For me personally, the rank was never a factor.
In Halo 3, I took my 50 seriously (the trek to it at least) and played seriously. Many of my friends played it too. I was never into the armor and whatnot, just rank and the thrill of teamwork and winning.
Reach destroyed the number of my friends who were playing Halo. Only a few remained, and with the change in rank and the rank not showing, I basically ended up playing for fun. The “randomness” with the abilities and whatnot began to work in my favor as I exploited the overpowered ones to play better. The sheer amount of customization in appearance just wasn’t my cup of tea.
By the time H4 came around, I was no longer a “competitive” player but was a very regular casual player. That is, I played Infinity Slayer a lot. I reached 130 soon and didn’t really care about any customization. I played for enjoying the game. Like Reach, there was more randomness than the previous iteration, but by now I had/have learned to utilize the randomness in my favor to make the game easier for me and exploit it to do well in Infinity Slayer games. I played regardless of rank and unlocks; that was never a factor for me.
But what will cause me to greatly decrease my time spent playing will be the DLC lockout. Now I cannot play Infinity Slayer, the basic gametype, right out of the box. I have tried to do Ricochet, and while the maps, pace, and game size are the same, it is a different experience with the obvious inclusion of the objectives. Big Team usually will be Slayer, but the maps, vehicles, pace, and game size are all off. Doubles is too small and too focused on teammwork (you can only really play with friends, otherwise you can’t have that level of teammwork). Throwdown is a level of competitiveness that I would have relished back in H3, but now, I don’t like having no radar (I can deal without camo and ordinance, fine, but the radar is my main thing), especially since nobody uses mics.
Look at the population overall. 5000 people (yes, 5 thousand) left Infinity Slayer and have not appeared on other playlists, whose populations have remained approx the same. The DLC lockout kicked out a lot of people because a lot of people just played the “basic” slayer game mode. Now I’m not saying whether this is right or wrong (I believe you can create a SEPARATE, NON-DLC slayer playlist alongside the current Infinity Slayer to fix this problem - again, not right/wrong, but I believe there is a more amicable solution versus a zero-sum game approach), but you have a lot of people who just simply left at that.
So I believe it is the DLC requirement on the main slayer playlist that has given the low population as of late.