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.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.
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.)