I believe Tor has a more active anti-censorship project than Signal does. While I don't think Signal is in any sense
OK with governments blocking it, the Tor folks have something along the lines of a whole anti-censorship
team (part paid and part volunteer). I don't believe Signal has a direct equivalent to that.
There could also be an element of luck or differences in how strongly particular governments are trying to block particular apps and circumvention methods at a given moment. I know that varies a lot from country to country.
Edit: However, Signal does have a "Censorship circumvention" feature under "Advanced" options. So there is some level of official work on that from Signal. Have your friends tried this feature?