It's usually one of three reasons: Denial, sunk-costs fallacy, or lack of knowledge on OSes
>"Apple sucks now, but I'd rather use it because I'm used to it and don't wanna bother with something new"
These types are usually non-/tech/ oldfags who just want a workstation and don't want to bother with learning anything new, they know when something's bad, but they don't have the will or self-esteem to do anything about it.
<"All those former Apple users complaining about their OS are a bunch of whiny manchildren, Apple is fine!"
Bitchy hipsters on a contrarian high, or people trying to defend what they like, as natural among people. Fanboys be fanboys, OS wars are the same vein as console wars anyway.
>"I don't wanna use windows, I'll see what these Apple devices are."
The least common type, but still a definite factor. Could be swayed to use GNU/Linux, but they simply don't know about it or if it exists.
<"iTS sOOO mAnY yeARS aGo or it ISNT thAT bAd"
[spoiler]They have it the other way around. MacOS was good before they became assholes and took out classic. Then everything went downhill. Then the downhill speed increased drastically when jobs died, because he could filter out the full-retard decisions people wanted to make. Mac isn't any better than windows nowadays.
T. Ex-macfag who left long before jobs died but is still surrounded by macfags.[/spoiler]