Canon seemingly likes to start their new systems of with an initial weird phase and get serious after a while. At least that's the impression I get looking back at the M, M2 and M3 to how far the EF-M system has come with the M50 and M6 II recently.
It's as if they decide on their naming scheme after the first couple of models come out. Clearly they've decided to parallel their DSLR naming scheme here...but the R and RP were named before that was decided on.
I think there was a hurry (not a rush, but "do something good, but be expeditious please") to get something out; that's not a ding on the R and RP which are plenty capable. They didn't have time to develop something totally new, so they did the best with what they had on hand.
The EOS-M series was originally completely uncharted territory for them, and a change of marketing concept too; they're no longer "dumbing down" full frames for the masses, but starting out to make a line of crop sensor cameras, and I think ultimately we will see some very awesome products in that line. (The M6-II blows away the other cameras I've owned by a huge margin. I took it and a Lumix point-and shoot on a trip; two days in the same place. I brought the Lummox in with me the first day and was constantly struggling to remember how to use it. The second day with the M6-II going back and rehitting the best parts of the place was a positive joy.)