Signal is useless for the only people it could be useful for. Most people don't need better-than-TLS security, and for those people Facebook Messenger or any number of other options are perfectly adequate. The people who do need better security are precisely the same people who a) can't afford to use phone numbers as IDs, not even burner phones, when they mean a hostile government who captures one person can immediately start tracing the location of all their contacts and b) can't afford to trust an unaudited server run by a company that could relatively easily be forced to cooperate with a hostile government (particularly when the encryption protocol uses a novel semi-TOFU construct that makes attacks a lot more plausible when the server is cooperating).

There are threat models where TLS is good enough. There are (mostly quite paranoid) threat models where GPG, for all its usability issues, makes sense. There is no threat model where Signal makes sense.

Finally, GPG's usability issues are contingent and fixable. Probably in GPG-the-program, and certainly without abandoning OpenPGP-the-protocol. Throw a bit of money at it, get an actual UX designer to spend a couple of months on a nice UI, and who knows where that would take us? Whereas Signals flaws are there by design.