Many music fans don't know much about Deep Purple beyond "Smoke on the Water." They were always bigger overseas than in the States, but to Deep Purple fans, they are the only band that matters. Their early albums were hugely important in shaping heavy metal, and most serious metal guitarists worship at the altar of Ritchie Blackmore. Much like Yes, they've had many singers and guitarists over the years, but the classic lineup (minus Blackmore and the recently deceased Jon Lord) has been hard at work over the last 15 years. They rarely hit America but in Japan, Russia and most of Europe, they are absolutely huge. Unfortunately for them, most Rock Hall voters live in America.
They were on the ballot last year, so hopefully their moment is coming soon. Sadly, Blackmore has made it very clear that he won't attend the event.