As much as I love all that Linux offers and working with it, the culture isn't there for Linux to be mainstream, and it's not even about feature parity or user-friendliness anymore. It's that the minimum standard offered by Microsoft, Google, and Apple is quite good and many of the annoyances that folks who do have a preference on OSes and such are not bothersome in the least bit to your average user.
The problem with positioning Linux as an alternative to Windows/MacOS is that there is a very real alternative (and has been for some time) of just not having a Desktop OS at all; do everything mobile. It might be strange for some people here, but Mobile-only is a very common and real choice made nowadays, and Android/iOS make this a pretty darn comfortable option. While the advertising and telemetry and crapware is even worse on Mobile, this leads to the other part of the issue where, based on my experience, the average user is completely attuned to and just accepts the advertising without question. 10 second ads randomly played as part of an App is just seen as normal; the barrage of randomly placed animated banners, auto-play videos that scroll with you, over-zealous sign-up requirements are absolutely the norm now as the big players have made this a reality, and many people just do not think twice about such practices. Youtube, popular news sites, and other such things have normalized these practices and now it's not even a concern for many people.
I've had this conversation with many people even directly in IT/Development related fields and I'm sure I come off as a Stallmanite-esque nutcase when I voice my frustrations about Google, for example. I purposefully avoid Chrome because of its awful auto-updater tactics on MacOS which basically took pages from malware authors on how to ensure the service is always alive and present on your OS. When I explained this decision to colleagues, the response was confusion more than anything, and wondering why I even wanted to stop this. (Neverminding their frustration with Google when it changed the login handling for its services)
Linux as an alternative __does__ exist, but this is only if you care enough to want to seek an alternative in the first place. Putting aside the monstrous mountain of issues with the non-Linux OSes, it's never been a better time to be a Windows/MacOS/iOS/Android user. The OSes are stable as heck, cross-compatibility is very good for virtually whatever program/application you want, the OSes run great on virtually all hardware with no configuration. The only consideration most of the time is tiny personal preferences. And best of all, for 3 of 4 of the OSes above, you don't pay a single cent (directly) for any of them. Windows holds out, but I cannot imagine this lasting much longer either.
With all of the above considered, the question just becomes "why would any of the mythical 'average consumer' even want to consider Linux in the first place?" As depressing as this may sound (and defeatist), as long as it gets them to Facebook and Youtube (and Instagram/whatever other social media they want), why would they care what does it and/or what that OS is doing?