You can't stack materia because even the same materia item can have different stats. Simple as that.
...which throws up the question: why are there different stats in the first place?

I mean, first they they do away with +1 and +2 drops and items because it's too cumbersome,
and the next week they introduce 4 tiers of materia with 7 subtiers each.

What kind of logic is that?!