I have the Leica R-E (x2!), R7, R8, R9 as well as Leica Ms.
Leica Ms are more popular because they are iconic. There are very few, if any, rangefinders that match up to them. I also shoot with Nikon rangefinders just because I am a camera hussy, and frankly they are cheap compared to Leicas. But there is a reason Nikon (and everyone else) gave up. The Leica Ms are so much better, so the Japanese moved on to develop SLRs.
My Leica Rs are fantastic to hold and use. They feel much nicer made than something like a Nikon FE, the ergonomics are perfect. Genius design being able to change meter pattern with the camera to eye, as well as meter mode by using one dial. Downside is that the lenses ARE expensive. Not as expensive as Leica M lenses, but waaaay more than Japanese equivalents. So while the bodies are cheap-ish - both my perfect condition R-Es were under $200, which is why I got 2!, a 50mm f2 lens starts at about $350.
The 50mm f2 on my Pentax slrs cost about $25... sure it is not as nicely made, but it still makes excellent pics and was only $25!!
So I think that is why Leica SLRs are not as heralded. People who are looking at Ms know the cost involved. People who look at SLRs tend to expect everything to be much cheaper. With Leica R series equipment, that is true relative to the M series.