A lot of the major developers of D4 left in the process. The people working there now are not exactly attached to the franchise.
They probably still have a lot of “source code” from D3 that they can reuse. It is cheap.
See they sold enough copies of D4. Now they can just cash in a bit on the “hamsters” that need a bit longer to realise the game has become what it is.
And then they move on to d5. They already said we will not need to wait another 10 years for d5.
And i bet when they start development they will once again promise a “back to the root” approach. A grim and dark fantasy horror game.
No thanks 