A lot of people switched to macs when Snow Leopard and Lion were current, so they got their first laptops with them preinstalled -- and obviously, they worked swimmingly (fresh installs, on the configurations Apple devs likely targeted during development).

Nowadays, thanks to Apple effectively deserting Macs for long stretches, less and less new people move into the ecosystem, which means the bulk of users are now upgraders -- who are bound to have more issues.