In the US, bank transfers are a pain. The credentials needed to send someone money are the same ones needed to take their money, and in any case, they often take several business days. So nobody uses them, instead preferring a whole industry of apps to do "that, but easier": Apple Cash, Cash App, Venmo, Zelle, all the features built into messaging apps.
On the other hand, the UK's Faster Payments system just works; you give someone your account number, they punch it into their bank's app, and the money goes through instantly. None of the aforementioned apps work in the UK, because nobody needs them. Bank transfers work well enough.
So the value proposition here is "Instead of sending someone a bank transfer, you can send us a bank transfer, send the coins to someone via Signal, then have them turn it back into real money via bank transfer. Also, be quick, since the coins are volatile and might lose value if you don't turn them back into real money quickly enough." I can't see how anyone would use that. (Sure, it might be more private, so people who really need it could use it. But Signal's whole value proposition is building privacy-protecting tools that are usable enough that they'll be used even by people who don't need the privacy feature per se.)
It's a feature designed by Americans for Americans, but released in a market where it doesn't make sense.
(Disclaimer: I'm am American in their first year as a student at a UK university. I think my understanding of the UK cultural issues here are decent, but I could be wrong.)