If you don’t think rap groups belong, then neither do ABBA or Donna Summer. What’s the difference?
Disco music evolved out of elements of rock to an extent, and ABBA borders on rock just barely enough at times in order for me to not be disturbed by their inclusion. If I were in charge of the Hall of Fame I ABBA and Summer would’ve been some of my last choices (I probably wouldn’t have thought to include them at all TBH).
Run-DMC bordered just enough on rock for me to accept their induction.