I think because stablecoins have to be fully backed, so its a big buyer of treasuries, while banks are just extending credit most of the time, so not really driving much demand for the base level debt, that's one reason for it

I can't speak for why people in the US would use stablecoins other than to trade in and out of tokens, but for a lot of other countries, USD of any kind is better than their local currency especially when doing cross border payments, and because these people feel that they can't "risk" the little they have on Bitcoin, they prefer the relative stablity of USD

You can see it in how fee sensitive they are and why USDT is so popular on chains like TRON or POLY so the theory is if you can give them something cheaper that is USD with LN, they might switch to the Bitcoin/LN ecosystem instead bringing along that activity and capital which will juice the LN networks numbers and hopefully convert these people to a sat standard in the future

Personally I think its more risk than benefit, as moving USD could be seen as money transmittors and stir up a shit load of drama, it also opens up LN to scams like fake tokens or shitcoins that people could fall for