I'd say cause Metallica is a bigger and better known band among the general public. They are more mainstream. I hear Metallica on local radio all the time, but rarely/never hear Megadeth. Other than Peace Sells and Symphony of Destruction (which I like both of, btw), I don't know that I can think of any Megadeth songs off the top of my head, but I could write you a whole list of Metallica songs.