I think you’re missing historical context. There are more options now, but when Signal came out (or became Signal, after TextSecure), it was the only tool to offer such strong cryptographic properties with its then novel double ratchet algorithm. Compared to OTR and, much worse, all the other crap that was not E2E encrypted at all, it was the first really credible option on a mass scale.
The crypto was reviewed by well-considered experts, and came out looking strong.
Telegram fought for years trying to say they were just as good and in fact better, which is entirely disingenuous considering it’s not an encrypted messaging app.
These things contributed to what you call the cult following. Which wouldn’t be negative (a cult film has a cult following) if not intended to mean “a cult like Scientology”.
True. With a lot of swearing andand not a lot of arguments though.