Linux has too many options. Mainstream users want to be able to get the same thing (or near it) on every computer they use. If I install Linux and you install Linux, are they the same? Maybe. But if we both install Windows or both install macOS? Yes. These options become intimidating. It's the same reason many people stuck with Windows XP until their computers caught fire or whatever other reasons they had to finally abandon it.

Gaming is also a factor. A better gaming story would drive up adoption amongst gamers and maybe spread to the people in their circles. But you'd have to have one or two distros that were particularly good for gaming that the majority of them used.