There are plenty of non-rock artists in there. It's really dedicated to music that's been in the mainstream American* culture in one way or another. That's as best as I can describe it, and with that thinking, DM certainly have their place in it.

*I'd really like to see them do a ceremony in London one year, or find some way to get the UK and Europe more involved. Rock & Roll started here in the 50's but if it weren't for The Beatles and everyone who followed them it would have stayed a fad. So many of the acts that are passed over for induction are English and doing something like that could make the HOF more legit in my eyes.