I suspect that ‘cheap’ is the problem. Agar does a pretty solid job for most gelling purposes(any edge cases people know of?); but you have to grow or harvest the appropriate, mostly marine, plants as feedstock. Some of the modified food starches avoid having to deal with aquaculture; but are more thickeners than decent gelling agents.
Gelatine has the advantage of basically being a value-recovery exercise on a byproduct of meat and milk production. Bones and skin aren’t free, or completely without value; but their production is certainly subsidized by the fact that they are(at least for now) a necessary byproduct of tasty meat. If there were no demand for that, raising animals for gelatine would probably be economically nutty compared to the alternatives.
