MMA isn't really a global sport the way something like boxing is. 90% of mma fighters are North American or Brazilian. Sure there's ONE FC and all, and a lot of good fighters coming out of places like Dagestan and Chechnya, but MMA isn't popular or respected in many parts of the world. Chances are, like most others, he just doesn't give a shit about MMA--and certainly not enough to take all the risks involved for such little reward.
Or a good amateur that didn't want to go pro. Why would he go pro in MMA if not boxing? What's the incentive? It's not really gonna happen unless that guy LIKES MMA, which many people don't. They don't really have good incentives in terms of money, definitely not compared to if they got into boxing. They have to learn entire new skillsets only to get paid less than they would if they kept boxing. It's more likely they would either quit combat sports altogether if they didn't want to go pro.
There's also the fact that no one has to pave the way for grapplers making the transition. It's already well-paved. There are gyms that specialize in teaching wrestlers how to fight. Strikers have only had scattered representation. The systems aren't in place for the transition to be fluid for them. Not many people are willing to be the innovators who show it can be done.
I definitely see your point about difficulty of learning new skills or not being interested in MMA altogether. Or the MMA not being a global sport yet (though this is really changing rapidly).
But I am not sure about this part - "They have to learn entire new skillsets only to get paid less than they would if they kept boxing." . I don't have clear data on the financial aspect of fighting in boxing vs MMA. Just whatever I see published here and there after some boxing and MMA events. And based on that it looks to me that middle-to-top tier in MMA can make more money, than boxers of the corresponding level. Say, somebody in top 15-20 in their division.
In addition, IMO, the following motivation can exist for a good boxer to try the transition - say, you are top 20-30 in your division, but you don't have a talent to make it big in boxing. Say, not fast enough or reflexes or whatever. But your standup is very good by MMA standards AND(!) you have some complementary gift/skill that can help you to become top fighter in MMA - say, you are very strong physically or you did ok in wrestling in college or both. So you tradeoff looks like to be top 30 boxer or MAY BE make it big in MMA.