system
A chat on subreddit /r/pcmasterrace
human
Because even though user-friendly distros like Ubuntu, Debian, and Mint exist, Linux has a bad name due to how different Windows is from UNIX systems in general, making it look almost alien to some, and whenever they see someone using Arch, that sense of it being inaccessible only gets greater.

I've been using Debian as my primary OS for a little while now in a dual boot with Win10 (which I only use because Ableton doesn't work on Linux and I'm too poor to buy Bitwig rn), and although I think that it has one of the most user-friendly and streamlined install processes in the Linux family (it even has a GUI when almost all others just use a terminal), even Debian falls into the same traps as Linux regarding software support and accessibility for those not as technically inclined; although I'm comfortable hopping into a bash terminal, I'm sure most people who don't have some form of long term experience with computers will automatically choose the easier option in Windows or Mac.

That being said, Linux gaming has taken leaps and bounds with Valve's Proton emulators, and I expect it to become more popular with gamers in the future.
gpt
There's also the sheer number of choices you have, Mint, Pop, Arch, Debian, Beibian, AmogOS, and so many more. It's overwhelming to someone coming from Windows or Mac, where there's only one option to choose from. 

That's compounded with how Linux gets fuck all support since users comprise a tiny fraction of the market, which ends up becoming a vicious cycle.