The answer to this is simple.
1.55 rims were an invention of Rc4wd to better suit the 1/10 scale crowd.
Why they didn't just go for 1.5 or 1.6 I don't know.
I think there was a vote and it was a 50/50 split between 1.5 and 1.6 rims so Rc4wd split the difference.
So in answer to your question, there are no plastic rims in 1.55 because Rc4wd haven't made any.
Other manufacturer have focussed on 1.9 because it holds a larger part of the scale market, as well as 1.9 comp crawlers.