I think it is because Apple had to twist AT&Ts arm to the breaking point to make iMessage work that way and then each carrier they added they had to force the issue again. Verizon turned them down because Apple wasn’t going to let them install crap and charge 10¢ (Or whatever) per iMessage in addition to SMS.
Google would have to convince all their hardware partners to do that carrier arm-twisting themselves and most those companies don’t have any leverage to do it even if they wanted.
Interesting, I did not know that. Out side the US market I think not supporting iMessage would be suicide for the carriers. If iMessage wasn't supported by one carrier, people would just switch. Maybe it helps that most carriers here have had unlimited SMS for a fix low fee for almost ten years.
Google would have to convince all their hardware partners to do that carrier arm-twisting themselves and most those companies don’t have any leverage to do it even if they wanted.