Either that, or its so that they can take turns being lazy. During some of the early stages in design process, this is what it often boils down to... (lazy in terms of modding, they may be very busy with other junk anyway) Of course, I have the excuse that brainstorming is a long process, but I've still yet to draw out a couple of the textures I already sketched out.. so yeah.
Concerning slopes, the issue is not only that you can't fit slopes into the renderer, but that even if you got them working they would not work anywhere near as smoothly as the rest of the game. Kind of like OGL, as cool as it is, it is far weaker in terms of supporting SRB2's level design than anything else, which was the reason why it was removed during release.
I have an all-or-nothing policy on including SRB2 features in my maps. If it works, then I use it... if it only partially works or only behaves under the right conditions, then I ditch it. (which is why I may have to abandon the cobweb gimmick in MFZ1) I don't want to take the chance on the experience being ruined by some random bug that I cannot anticipate (or worse, can anticipate but not fix).