Carriers would need to change the way they handle SMS, and everything a carrier does is subject to state regulations. And states seem to like clear text.
Why's that? I understand the message would increase in size because of the encryption, but I think it would be technically feasible now. Didn't even apple just introduce encryption into their messenger? My issue with apple's encryption is it's closed source and apple only.
Apple's iMessage just detects when the other party is using an iOS device and sends them an iMessage instead of an SMS. The GP is right; cell service is so aggressively regulated that there's no hope of carriers adopting a better standard, so the best we can hope for is that at some point mobile OS distributors agree on some open standard like the Signal protocol.
iMessage end-to-end encryption is not a recent introduction. It has been in place for several years, though I can't find the exact iOS version it was introduced in.