To me it's not that the fighters are less skilled, or less recognizable, or less exciting. It's that cards feel, on average, more meaningless in terms of what or does for divisions. It felt like the main card would have 2 or 3 fights with a ranked person on it and the main was usually 2 people fighting, the winner of which was set up to be in the title picture for it. Only rare fight nights or international cards lowered the level of quality so that the main didn't have some immediate impact on a division
Just did a little experiment.
I randomly went back 7 years to
2018, there was a fight night event on 3/17. Edwards was #15, fighting on the main card. He was on a 4 fight win streak, so I'm not sure why he was booked against a guy unranked, but that's still someone in the rankings working their way up. Co main was #11 Jan vs #4 Manuwa (who had just lost a title eliminator, so you know this fight had some meaning to it). This fight got Jan into the top 5. He wasn't in title range yet because of where they were both at beforehand, not exactly streaking guys and because Jan was already getting a huge leap with this fight anyway, but after this, he fought and beat Krylov, and then got a title eliminator against Santos from there. The main was #8 Volkov on a 5 fight win streak (3 in the UFC) beating #3 Werdum on a 2 fight win streak. This directly led to Volkov getting a title eliminator against Lewis.
That's what an event should look like. Main event winner got a title eliminator. Co main got put into 1 more fight before a title eliminator. That's meaningful.
The fight night card this weekend 3/15 is headlined by #8 Vettori off a loss vs #12 Dolidze in a rematch from just 2 years ago. They're the only ranked fighters on the whole event, and that's not even because of any pullouts from any intended fights to my knowledge.
The winner of this "main event" won't be in a title eliminator, nor even near the top 5, and no one else on the card is going anywhere any time soon. That main event isn't even good enough to be yesteryear's co-main.
If you were interested in who was going to soon challenge for the HW belt, or what prospective top fighters were going to emerge at LHW back in 2018, you watched that fight night. If you care about what's happening at MW right now, you can take a vacation on this event, it's not important and the results won't matter for nearly 2 years.