Having more region codes means you need more types of players, more hassles with people in border regions who want to be able to play both, and more agreements. My guess is that they streamlined the Bluray codes to fit current economic realities. There’s an argument for using the same regions as the old system, but you don’t want to lock yourself into those regions forever if things change.
The wikipedia page suggests that 70% of major studio releases aren’t even using the region code. Perhaps they decided it was more trouble than it was worth.