SMS is very rarely used anymore outside of the US and a few select other countries, and for most people Signal and similar messengers serve as a replacement for text messages, not a supplement. It's probably just not a worthwhile market/feature to keep anymore.

Come to think of it, I'm having a very hard time coming up with any sensible use case for using Signal over text message. Presumably both sides still need Signal for the encryption to work, so what's the point? Might as well use the internet to send the message. The only scenario I can see it being useful is when you have GSM, but no internet connectivity, and that's rare these days.