Trendy, popular, whatever... The original comment is spot on. Products becomes trendy and popular mostly because of great marketing. The reason you prefer OSX (or almost anything else in your life) is because you've been conditioned to. We make decisions based on a complex cocktail of facts and feelings, most of them external to our own thinking and, therefore, susceptible to clever manipulation.
Every BMW owner buys an "ultimate driving machine" (with an automatic transmission) and most OSX users believe they're enjoying "the best UI/UX" (and obediently squeeze their life into ~500 vertical pixels leaving the rest to the Dock and global menu).
It does not make sense to look for a technical explanation for why "Linux desktop" is not popular. Technology has nothing to do with popularity. Moreover, Linux Desktop is indeed widely popular among people who're pre-conditioned to like things it offers.
Here's an anecdote: I have introduced younger relatives to computers in mid-2000s using only Ubuntu. After years of daily Gnome 2.x use they find both OSX and Windows incredibly "retarded" and hard to use.
Don't kid yourself thinking we're far ahead of Pavlov dogs.