Its just the passage of time. Even the most legendary artists in Hip Hop or any genre have a 5-10 year window of their prime, and after that for the most part their new music is for the die hard fans. Either they try new things and alienate their fanbase, or keep making inferior versions of their prime years, and people still lose interest.
Of course some artists sounds age better than others, and can adapt easier to their eras, but theres always a limit. If you see a thread on here about any 90s producer theres always a bunch of people saying they suck now.
Alchemist is still up there to this day. Madlib too. I don’t think it’s a magical well of talent and ability that suddenly dries up because an arbitrary time frame has elapsed.
I think for some producers they get money and a lot of other distractions which impact their craft…
Partying, drugs, alcohol, women, having kids, other business interests, lawsuits, stalkers, a myriad of other pressures of fame. Straight up mental health issues, and also general health / disability problems as they get older (you just wouldn’t necessarily know it as most are invisible).
For Dre it could have been a number of the above reasons.