Because money. They want to sell new controllers at $70 a pop. These are businesses we are talking about. Businesses care about money.
I’m guessing letting the ds4 work with ps4 is to appease the “I used to be able to play this with four people on my ps4 but now I can’t on the new system unless I spend another $210 on controllers.“
It’s better than only allowing ps5 controllers no matter what, and the first system I’ve know of to allow backwards compatibility on controllers. Not that it’s something I’ve paid attention to, so maybe someone else has done it, it’s just the first time I remember seeing it.