in most case it will be what you most do in each mission more.
and to learn from the mistake’s you make and not going to make if you do it later on legendary.
for exemple in halo 5 when you fight against 4 Warden Eternals for the first time to see it that can become a problem when you do it on legendary more
I feel attacked. Stop pressuring me, I’ll get it done when I have the time. 
Seriously tho, back in the old days I used to jump straight into Legendary immediately after picking it up from a midnight release. Nowadays tho I have like kids to take care of so time is a hot commodity. I play for the story first (which was awesome btw), and I’ll go back later for legendary/laso. Eventually.
Given the game’s F2P status, a large portion of the percentage of people who have played the game don’t own the campaign. I think that’s gonna skew the achievement completion percentage quite a bit.
I’d say it’s more the lack of co-op and mission replay, and the campaign just being overall mediocre.
I played through on Heroic but can imagine most people would play on Normal because of the bullet sponge bosses.
The game was kind of a chore too. I didn’t enjoy it as much as it was hyped up. It was a 7/10 for me and maybe that’s a little generous.
Because infinite campaign doesn’t have that Halo experience. It was treated more like Zelda.
This is Halo, not Zelda 
Vet here. Playing on normal. Don’t have the time anymore to go Heroic and Legendary. Maybe down the road after I finish on normal.
All / most of your vets are in 40’s with kids and houses. For me personally I don’t have time to invest in grueling campaigns in any game that takes too long. If it does I’ll just put controller down and work on the honey-do list lol.
Don’t forget that for infinite they said they had changed things, and whereas they normally started with Heroic then changed things for normal/easy/legendary, with Infinite they started with normal then changed things for easy/heroic/legendary.
With the length of the game, I’m not surprised so few have beaten it on heroic. I can easily beat heroic on most of the other games in just a couple of hours, Infinite takes much more. Infinite also leans much more heavily into boss fights, and some of those bosses take quite a few tries on heroic. I can see how someone would decide to give up after being squashed by boss battles a few times.
It’s actually pretty common for games to have low completion rates especially on higher difficulties. I would figure most people played Normal, not heroic/legendary. Couple that with the fact the game is on gp and pc, there’s a lot more accessibility compared to previous entries.
Because the campaign wasn’t worth playing twice… I’ve been playing halo since the beginning, and this just doesn’t feel like a halo campaign. Played it once on normal, thought it was just as bad halo 5’s campaign ( in many cases actually worse…). Few moments of brilliance, but about 30% of it was garbage… 20% was good, 10% was brilliant, 20% was just playing 5k simulator, and 20% of it was straight up missing! This is the first halo game that I’m glad i didn’t actually buy. Soo many missed opportunities all because they wanted to make an open world that feels empty and boring. The core multiplayer is amazing but has no content, and it feels like they put more time and resources into the store than the actual multiplayer… All in all 343 fails once again… infinite’s multiplayer will probably be in a good spot on like a year or 2. Just like they did with MCC and halo 5. You would think 343 would learn how to launch a freaking game by now.
I didnt enjoy it much. Too short, with a lot of empty space walking, cant replay missions either.
here are some things you miss.
with the classic campaign mode you cant update the campaign mode any more that means its staying for good the same thing always over and over with notting news.
in the open world you can add new things in it and update the campaign mode with new missions and place’s.
look at zelda the developers told there that with the basic thing there cant chance things or adding new things in the game.
but with the open world there can bring out DLC’s and update it with new things there come up later on.
the same is with halo.
if after some time the developers have come up with something good to add in the campaign but with the classic way there cant do it any more but now with the open world there can do it.
To me one of the reasons, in addition to those already pointed out by other users (players don’t purchase campaign, others wait for coop etc…), is the lack of rewards for playing on higher difficulties.
All cosmetics (coatings/emblems) can be found by playing on easy and (again) there’s no Mark VI armor for completing the game on legendary.
PS: I’m one of those “vets” who actually enjoy playing on legendary/slaso, and I find legendary difficulty in Infinite pretty easy.
Im waiting for infinite to go bargain bin where it belongs, or 10$ off of some kid on FB marketplace.
The only way i’d consider getting it is through gamepass, and the value isn’t there for me to get gamepass yet.
Psst… That was H5. No worries though. Most people have tried to erase that game’s story from their memory.
sorry my fault i have chance it.
its been a long time since i have play halo 5 campaign again and much much longer since halo 4 campaign more.
I did my part. Had Legendary beaten in the first 3 days… Talk about difficult…But even then, definitely not worth 60 bucks!!! 343 you greedy succubus.
Bro, with GP you didn’t have to pay 60, you’re a fool just like everybody else who paid that much for a campaign that also lacks mission replay and co-op. Even if it had those, the max to spend ever would have to be 30. Congratulations, you played yourself. Another one who literally has no ideea how to properly spend their own money
Halo has never been that kind of campaign… its always been a replay mission campaign. you’re in the mood to play a certain mission on a certain difficulty. you or you and a buddy sign on and go to town. NOW if you want to replay a specific part of the campaign you have to restart the whole freaking campaign. and your talking about how you can expand an open world. you can do expand it, too bad that’s what most of us OG halo fans hate. The the small missions that just have you running around playing errand boy are boring as hell. if you want that go play destiny. Give us the big missions that actually have meaning to the story with epic cinematic cutscenes. That is halo.
do you not have read what there have told about it.
that there are looking how you can replay the campaign missions on the same save fille and thats something that needs a lot off time.
since you have to find more how do you replay the same missions since its not that easy as you think.
since some off the bosses are death and some off then like chak 'Lok and bassus need also a solution how there can respan back on there location again and same go’s for the drill that you have destroy all.
or the spire that has is shut down to make it active again.
most off you OG halo fans wane see more that halo stay’s on the halo 3 road and that notting new can be add that each halo most become a halo 3.x version more the same old thing with almost notting new only the campaign but thats all.
i can also tell the same thing to you.
if you wane play mission for mission then go play halo 3 or so.
and for how long do’s it not take to get bored from it and you stop playing the campaign since its always the same thing again and again.
now you can do things on a lot off way’s like make your own challenges from it.
like compleet the game on legendary without using any power weapon.
or compleet it with only a pistol.
or use melee only.