a) Never heard of G/L
b) Don't know how to install it; don't care; 'Windows/Mac is good enough for me'
c) Have had compatibility problems with it when they tried
Of course, the idea of general population depends on who you interact with the most, but let's assume general means people who go to Best Buy/Costco/Walmart for computers. These people would probably be willing to try G/L if it came by default. Yet the moment some weird error came up that involved anything more than a simple Google search, back to the store the computer goes. This is not only a loss of money for sellers and manufacturers, but also ends up being really bad PR.
G/L distros like gNewsense or Trisquel (both of which run Linux-libre) would run and sell really well at Best Buy if the video cards, wifi adapters, etc. they would come with worked out of the box. Yet God forbid you use some other [wifi adapter, insert other unsupported device] that doesn't have the right firmware! (Of course, this also applies to regular G/L albeit much less so.) Most 'average Joes' don't care about things like FLOSS unless it works and doesn't require a lot of work to setup and use constantly.
That's not to mention running Windows programs that don't have a FLOSS alternative.
My point is this: G/L won't become immensely popular without the major compatibility issues being fixed and it becoming a default install on a whole major line of computers. Compatibility won't be fixed until the vendors of the devices or programs see a significant profit intake from releasing the firmware/whatever and do so. Default installation won't happen until some major hardware company comes along and sells G/L only.
TL;DR: G/L needs major support to become commercially popular.