Perhaps a better question is: asking why Apple isn't mainstream?

Linux almost always needs to be installed, whereas Apple is plug n' play. Plus Linux has a reputation of being much more complicated than it actually is.

The disparity between the proportion of iMac sales vs the people who could afford an iMac is rather enormous, but I have this idea that for iPhones and Androids, this is reversed.

I find that conundrum, assuming it's true, kinda interesting.