Well, for one thing, we know that MHA's first season didn't make quite an impact as much as we hoped for since it only adapted the weakest arcs of the manga. AoT however, blew up almost instantly as soon as the anime aired, and ever since then it's been going really strong since.
Now that its 2nd season is coming up, there is hype riding on it whereas MHA doesn't quite have as much, I believe. Even then, it's only constrained within the ones who read the manga already. Newcomers though? Not so much IMO.