May be because they don't do much of it and are uncomfortable with it. My neighbor across the street is a roofing contractor. He just re-reoofed his house and garage with metal, so I got to watch and see how it went.No one recommends metal here-of 3 guys I've spoken to now.
It went slow. Lots of things the crew were used to no longer applied, and they were pretty frustrated. He did his own house to find out how metal went (it was his first metal job) and he found out the hard way. I think it took a couple of weeks to do, whereas a shingle job would be 3 or 4 days. It looks great, though, and he's pretty happy with the result. I'll ask him what his overall opinion is the next time I see him.