p2p exchanges are not more difficult to use than non-custodial LN wallets. you aren't seriously suggesting that p2p is an insurmountable barrier compared to managing payment channels.
Discussion
I don’t think p2p is too hard but is it widely known for them to do the work to do that? No. They’ll just find some other shitcoin on exchange before they go learning about buying a coin on p2p. I would also make a counter point to your argument about this being similar to non-custodial lightning. It’s not even close non-custodial lightning wallets are pretty new, monero has been out for years dwindling volume.
Monero isn’t a common neutral good so it has a salability issue. If you ask most normies if they’ve heard of bitcoin they’ll say yes. Monero would get a small fraction of “yes” to that question relative to overwhelming “no”