It's a lot of reasons.
I think just a big thing is the barrier to entry is lower. Every mf with a laptop can make music now. Even something like having to book studio time was a hurdle that meant you had to be serious about this shyt.
And additionally you've got all the corners being cut, things like ...the punch ins, the "producers" who ain't producing shyt, just recording punched in verses one time, no real musical vocabulary. None of these mfs listen to anything but rap music of the past decade. Ghostface was taking influence from soul music, Rakim was listening to jazz, etc etc
And then there's the death of artist development at the labels.
It's death by a thousand cuts, there's no one reason but the bar has been lowered and lowered and now due to streaming most people treat music as disposable entertainment. And that includes the artists themselves.