Least path of resistance plain and simple. They want to maintain their dominance without risking support of carriers, manufacturers.

I don't know if I'd call it the path of least resistance as the other commenter did, but it is definitely about how to control the default messaging app without pissing off carriers (in particular) and OEMs (to a lesser extent). They could have strong armed them through the play services agreement, but they decided the carrot was better than the stick here, even if it meant making some compromises.

This doesn't make sense to me because (AFAIK) carriers and OEMs are free to set any messaging app as the default.

Wouldn't it be better for Google to focus on making users happy? And shouldn't user safety and security come above all of that?

RCS feels like something from a long time ago.

They tried to make users happy with Allo, it didn't work, so now they're trying to use their platform leverage. One thing I'm not even really clear on is why Google cares about messaging, but I'm guessing it's more about Android competitiveness than anything else.

They didn't try very hard. I had never heard of Allo until a month ago when I got a Pixel 2 and saw the app pre-installed. If they really wanted to push it, they would have made it the default SMS app.

It seems odd to me that Google has been unable to solve a problem that Apple solved in 2012.

Allo also doesn't have encryption by default. It's considered only in "incognito" mode.