Honestly I don't think the UX is there for exponential growth yet. Setting up your own LN node is fairly technical even for engineering types, but it's getting better all the time.

Slow and steady growth is fine for now, exponential will come when there's a widespread demand/need for LN. Honestly I expect things to be just "good enough" by the next bull run for some big adoption.

Ordinals are pushing up onchain fees a bit. But fees are still sub-$1 that's not enough to get people to change.

People are spending their Bitcoin more than they used to, but not much and alot of that is online. If it's online, it doesn't really matter if it takes two seconds or two hours. So Bitrefill, one of the biggest actual venders that sells things for Bitcoin gets it mostly on-chain.

Multicoin exchanges don't want to spend time figuring out running a node or balancing liquidity. (They also might not want to make it easier to withdraw at all.) Afaik, Kraken is the only multicoin exchange that supports Lightning. If a bigger exchange, such as Coinbase or Binance (the biggest) it would probably do a lot.

I'm gonna say it too, people think it doesn't work. Their is actual incentivizes from altcoin publications pushing this narrative. In 2018, a self custodial LN payment did have a 50% 1st try failure rate though. Doesn't matter if that's no longer the case.