I'm going to pull an example from the Quake gametypes here, because that's what will make sense to you people about the gametypes'.
In Quake Live, CTF is not the most played game type (its second most, not including duel) Why? Because of the complex tactics. Clan Arena is much more appealing to newer players as you already have all the weapons, and the objective is to just kill everyone. Simples.
I'm not sure if anyone around here has released, but CTF is not fun if you have never played it before.
Even though I had played legions CTF before, when I started Quake CTF, I was bored with it, quite quickly in fact. It was because I didn't know what to do apart from get the enemy flag, and defend ours. Actually lets rephrase that, I didn't know how to accomplish those goals using the quake Gameplay, even though I knew what to do. This is the exact same thing in legions. On the Old IA help button when playing the game, it just said go get the flag and bring it back home, nothing about the ways you could do it.
This goes for the other gametypes as well like Duel (Which I've only started to get reasonable at now) and TDM, which I still have no idea how to win.
I'm gonna keep it to that, otherwise I'll end up ranting.