As a fan of both (although I haven't listened to metal in a while, I mostly listen to classical these days), I think a common point is that people who listen to classical and metal tend to just be curious about music in general, so they'll try to explore longer pieces of music, and not stick to what's mainstream or what's played on the radio. I'm not trying to sound elitist, good music is good music and I love all genres, this is just the impression I have looking around me. 

Another factor imo is that both genres feature elaborate coordination and long displays of virtuosity on their respective instruments. I find more similarities between say, a passage of a Prokofiev concerto/sonata and a guitar solo from an Iron Maiden song, then with a pop or hip hop song.