Because supercomputer owners know what they're doing and how to make the most of their system.
Most standard users just want something that works and supports their commonly-used programs.
Not everyone enjoys playing around with alternative software and OSes, and most workplaces I've been to use Microsoft products because it's the standard.
eg. If Photoshop works for them, they see no reason to give GIMP a chance.
Why spend time on something that may work when they already have a solution that does work?*
I think Linux will see more standard users with steam for linux, the ubuntu phone, etc.
This far down the track I think it's moreso about exposure and information.
Most people I know have never even heard of linux, and those who have automatically assume it's too difficult for them to use.
*I don't think like that, obviously. Otherwise I probably wouldn't be on this forum. But it's easy to understand where such people are coming from.
PC: LMDE x64 MATE/Cinnamon, Kernel 3.9, Nvidia GT620, i7 3770K, 16GB RAM
Laptop: LMDE x64 MATE, Kernel 3.8.6, i3, 6GB RAM