Maybe the same reason a 2 party system does well in the US. People are not intelligent enough to make their own choices (and they get overwhelmed) and want "daddy" to tell them what to choose, much like how Windows is just "the thing you use on a PC". People don't even want to find their own music anymore, they want the Al Gore Rhythm to feed them everything.

That's why when people ask me on Linux I just say Mint, use default settings. Because they won't understand anything else and Mint will likely work fine in 98% of cases.