|Demetrious Johnson current flyweight champ|
What brought this up was watching UFC on Fox 6 back in January. Where Demetrious Johnson and John Dodson had it out for the flyweight belt. The fight that transpired was one of the most exciting back and forth fights I've seen in a while. Yet, since most people believe "heavyweight equals strongest" most people don't care about seeing the lightweight guys. It's something that's been around since at least pro boxing has been around. So, people probably won't buy a pay per view if it's headlined by fighters of the lighter weight classes.
Here's what you should know about the lighter weight classes if you're not familiar as to why they ought to be given the chance to headline main events more often. The lighter fighters are able to move more often and faster and usually thanks to how well their cardio is they can fight with high energy throughout all the rounds. Going back to the Johnson vs Dodson bout there wasn't much of a lull throughout the fight, though there were some illegal blows that stopped things for a bit, but that doesn't count. There's also fights in the 155 pound weight class like the famous Anthony Pettis vs Ben Henderson where something that is much less likely to be seen by heavyweights, a kick by jumping off the cage. There's also the fact that the 155 lightweight class is a division full of top level fighters, another reason why you won't be bored.
Going on the subject of heavyweight fighters and the heavyweight champion being "the strongest man on the planet" as I've said earlier has been around since at least pro boxing has been around. People would think the biggest guys were naturally the strongest. Of course you can't fake physics, having more mass especially muscle mass does make one physically stronger and there are weight divisions because of this, but that isn't to say someone like Anthony Pettis couldn't defend himself from a heavyweight if need be. Mixed Martial Arts is about technique not just brute strength.
In short, watch the fights of the lighter weight class guys (and women too) and see for yourself how exciting and main event worthy they can be. The athletes may be smaller but they're fights are certainly not.