On one hand I have to assume they know best and they simply decided other environments won't work well with the formula. On the other I indeed question the need for having at least two different catacombs in Dark Souls III, etc. DS1 seemed to have a lot more crazy and varied environments. Perhaps as the games become more detailed and more expensive to make, From feels more averse to taking risks. The definition of risk implies that it may not pay off, and players have been pretty vocal about things like Lost Izalith, or Blightown's original frame rate.
I think they're intertwined issues. DkS3 is a totally fine game, but I think it's extremely dull. A greatest hits of everything they'd done at that time, and that was already four quite similar games in quick succession. But the fact that it's in the same place means they feel compelled to literally have the same locations, so you can say 'aahhhhhh' constantly.
The whole poison swamp thing, or starting in castle wall bits, or dark, shitty horrible wood plank towns are so played it's unreal. A lot of it is just practical though, even when you're not reusing assets, which is a lot of the time, you don't need to reinvent everything, as you would with entirely new regions. The reality is that Demon's Souls wasn't in the same place as the Dark games, but it still features all the environments those games do, so maybe the region wasn't the issue, it was just the amount of time they had to envision something truly new.
I gather that between King's Field and Shadow Tower, a lot of those environments actually predated Demon's too. It's a lot to ask for games to be made in such short a period, and be radically different. I guess in that sense, more than BB2, what I really want is them to take a more lengthy break between projects in order to have the time needed to wow players with really fresh locales. For all I know, Sekiro is that after some breaking point in the game, we shall see.