Because it's still theory 7 years after announcement it has been proven again and again to not be a real alternative.
The reason adoption has been so low is that with current hardware you cannot run a full node in a phone, so you either get a server to run a full node or you trade-off some control/security of your coins
There are still other theoretical problems, mostly things like denial-of-services, but adoption has been what has stagnated it.
The hardware/control tradeoff is a real PITA, we need a phone with like 50x more battery or efficient to be able to run lightning in a completely self-custodial manner
And there are still some people advocating for blocksize increase, which will make the problems of lightning even worse