Missing Playlists and/or new Maps

A friend of mine and I have played Halo 5 from launch till Christmas, after that we switched to other games. Every month when a new content update dropped, I hopped into the game to see what was new, but nothing got me hooked for long (Fiesta and Assault were awesome though).

We decided to wait until all the content drops would be available, so we can play everything at once. To have the full experience.
So, Firefight came out and my friend and I really love it, it´s awesome.

And then we thought: How about a few rounds of Arena? With all these new maps there should be a great amount of variety, right?

I don´t know what happened, but where are the new maps? Every map we played was an old one from launch, and I´m sorry but they are all pretty meh`. In custom games we found all of them, but not in Arena. And to my surprise, Fiesta AND Assault are gone as well.

So I did some research on Waypoint, and some people said that these maps are plain and simple not in Arena at the moment… is this true? Who brings out a lot of DLC only to take it out of the public playlists? Assault is gone as well as far as I know, and Fiesta is nowhere to be found. (Also, Slayer in Team Arena is pretty unnecessary imho, it has its own playlist).

As it stands now we only play Warzone, because Arena is the exact same like back then at Christmas (Molten got added, so that´s a start i guess).

Don´t get me wrong, Infection for example is awesome and I love Halo 5, but why hold content in form of maps/playlists back from the player base?