Developing material for a system is the same as developing a system. Every new spell, critter, feat and feature needs to be cross-checked against the others of each and every system to avoid abuse - unless you don't care and just want to churn out a crappy product.
So, you'd either be asking a group of developers to learn 4-6 fundamentally different systems, and balance everything they do in those systems (which - not happening). Or, you hire a bunch of different teams, which is more expensive.
Besides, each time they've switched systems they've essentially said "this system's flaws are so great that to continue we need to redesign the game engine." Why would you go back to an engine that you've already stretched and mutilated to the point that you couldn't add anything more to it.