I'd quite like to see Awkward Zombie here. Honestly though, the reason why we don't see comics like Awkward Zombie/Penny Arcade/XKCD/Cyanide and Happiness/etc. here is for three main reasons:
1.) Most of those comic artists want to stay independent and don't want to get involved with hosting their comics on third-party websites, especially considering most third-party websites are low-quality and they want to keep their series independent with a single source of fans.
2.) Quite a few of those comic artists are already pretty well off in terms of monetary support from fans, either through sold merchandise or ads or donations or Patreon, or whatnot. So Tapastic might not make much of a difference.
3.) They might just not have heard of Tapastic It's still relatively small in comparison to other sites that have been around longer (DrunkDuck, SmackJeeves, etc.)
We do have GaMERCaT though, which started out mainly on its website and gained most of its popularity there, then ended up here. So this doesn't apply to ALL of those famous comic artists, but I'm sure the above three reasons may be taken into account by some of them x3"