Ay... in the late-90s/early 2000s, all the older artists learned what every artist before them had to learn too: their time was over. Wasn't necessarily their fault, wasn't anybody's fault. It just happened, like it always does. No matter how great something is/was, there'll be a time it becomes the old way of doing things, and something else will replace it.
I'm sure there were people from the Motown era that LOVED that sound, and hated 80s R&B and ESPECIALLY rap. But that Motown sound, no matter how classic it is, was gone by then. And so was the 70s soul era. So the old school way of hip-hop also changed and became less relevant. The sound/style switched, it's switched since then, and it'll switch again.
There's no real reason for WHY it changed, it just did... everything before it changed, and it was no different. "Real hip-hop" was another certain era/sound that wiped out something before it, and then other things wiped it out..
. and someday "trap" shyt won't be poppin' anymore either.