The whole thing changed over time.
It was a series of small steps, most having to do with international expansion.
If you go from a roster of 100 to a roster of 700, the quality is going to drop, same with going from 10 events a year to 50.
I'm old. I remember when a fight card would come out and I would know close to every fighter on the card, or at minimum one of the fighters in a bout.
2005 or so, if you followed the sport, the entire UFC roster was basically the guys that currently fill up the history books. Matt Hughes and Chuck Liddell fought on every other card. Prelims would be guys like Shonie Carter, Chris Lytle, Karo, Din Thomas.
Also if you look at the cards, it was often 8-10 fights. I think the recent Aussie card had 15.
Different era. Back then we complained that we're no cards for another month, but the events were solid and worth the wait and there was a lot of talent in other orgs to fill in between.