FWIW I believe this is because R is very hard to compile (lots of dependencies like graphics, Fortran, etc.) -- and one of the R core devs is also a Debian deve

FWIW I believe this is because R is very hard to compile (lots of dependencies like graphics, Fortran, etc.) -- and one of the R core devs is also a Debian developer! The packaging is a lot of work. That is, it helps to have the same exact dependency versions as Debian, because that's what the core devs test with. This is a flaw in the distro model IMO ... The other distros basically have to copy what Debian does, and do it imperfectly as far as I know.