Because cryptobros desperately need a legitimate reason crypto is the future, and thus valuable, and definitely not a round of tulipmania.
The argument isn't only made by cryptobros though.
Banks seem to be pointing at the same need and I just don't see why. At best it gets around regulation if a stablecoin isn't treated the same as fiat. With either, though, a bank can already create and give out whatever they want and track it in a database (all be it a more standard db and not a blockchain or public ledger).
Banks are run by finance people with a profit incentive. They're susceptible to trends - see subprime lending and CDOs in 2008 where even the big boys who knew it was a bad idea eventually started doing it.
That still doesn't do it for me though.
Banks in the US are somewhere between a legally protected monopoly and a racket. They would only make more money if they can charge higher fees or convince the economy as a whole to make more or larger transactions.
If banks could do either of those with a stablecoin they could do it with USD, which again is effectively a stablecoin that lives in the banks' private databases.