Lightning is popular because few prefer slow transactions.

El Salvador primarily uses lightning network, same network Strike uses for instant cross border payments.

Of course, I stand corrected about that.

So, aside from irreversibility of transactions, what is preventing adoption?

It doesn't even use that, the Chivo wallet only uses LN for external transactions, everything else is a plain old centralised app.

I don't have time to thoroughly research the topic and come back to you, so maybe you can do some leg work.

But I do know people over estimate what can be accomplished in a year and underestimate what can be accomplished in 10.

also correlation doesn't equal causation, the Overton window, etc

Setting up lightning channels are still slow, expensive, and worse require hot, online wallets.

Those that need unstoppable transactions are willing to pay the 10'000 satoshi fee on a transaction with arbitrary amount.

Which users actually need and want irreversible transactions. What's the utility of a system that doesn't let you combat fraud?

A system with a middleman capable of reversing transactions enables fraud. See also: the folks behind Flipper Zero being permanently robbed of more than a million USD thanks to PayPal's whims (as the latest of a long list of high-profile cases of PayPal taking vendors' money and running).

people targeted by law enforcement or secret police need unstoppable transactions