It's could be a possible reason is we are still at the early age of crypto, people are still confused about what is crypto.
I think that's definitely part of it. Guaranteed when the younger generations come of age, there will be increased adoption because they'll have known about it all of their lives--even if they never end up using it as a currency, they might still own it just to own it. The same can't be said about older folks today, or even most people in their 40s (with certain exceptions, AHEM).
The biggest factor I think is the lack of necessity for owning crypto. For nearly everyone, fiat works just fine and people have no problems whatsoever using a bank card or cash or their smartphone to make purchases. Who in their right mind would spend fiat to buy bitcoin simply to spend bitcoin? Yeah, if you were buying something on the dark markets that would make sense, but nobody does that for purchases that can easily be made with fiat--and they shouldn't IMO. It just makes no sense.
If the world ever gets itself into a global hyperinflation scenario--real SHTF stuff of nightmares--then I could see where bitcoin would become extremely useful, and no doubt people would adopt it as a form of money en masse. But unless or until that happens, bitcoin is going to stay out of the mainstream, at least as a currency. It's already coming into the mainstream as an investment tool (which is fantastic, btw).