For the same reasons the world is ok with only 2 commercially viable mass market desktop operating systems. Consolidation benefits users. It increases capital investment in the dominant platforms making them better faster; it increases the chances your software and skills will be compatible with your next computer; you benefit if your family, friends or colleagues use a compatible platform; it focuses developer efforts to have fewer platforms to develop for so there’s more better software. Users flock to dominant platforms because it’s in their interests to do so, and in their interests that others do too.

This is why desktop Linux never gets anywhere. Even if one distro was dominant in users, that’s completely decoupled from it getting the lions share of developer support and it wouldn’t give it any advantage in resources. There’s no feedback loop to elevate a dominant distro. Maybe that’s a good thing, perhaps the value in desktop Linux is it’s diversity and ability to address niche specialisation, at the price of market power.

There is a feedback loop in commercial server distros because that is a commercial market, hence RedHat’s dominance.