because using only Vanilla resources is much the same; you're not adding anything new, just using stuff already made.
Which ironically, is far from the entire point of ZDoom in the first place. There is a reason for the creation of DECORATE, Randy And Graf Zahl didn't put their efforts into it to have people complain about it.
Besides, what with all the Quake features (such as the console), do people honestly expect that the appearance of new monsters, items, weapons, etc. wouldn't come?