I have thesame problem with a lot of my buddies here in Canada. During high school I too enjoyed listening to Metallica and Iron Maiden, but even at that time I enjoyed listening to music that was a little harder to find. The problem is that by reading the promotional material for bands like Metallica, they don't mention any contemporaries that have broadened the scope of the metal genre.
So people aren't exposed to it.
And these guys here are fuckin stuck recycling their old Metallica tapes and Maiden tapes, cause there is nothing quality coming out any more, and the only new music they've heard about is the stuff on Ozzfest. It's saddening in a way.
Of course I try to get them into stuff like Opeth and Hollenthon, but they're just not ready for it. It's like eating meat and potatoes everynight for years and years, and then switching to burritos.

And the folks who can't even get into meat and potatoes have even less of a basis from which to judge the burrito.
It's a shame that the world in general fails to recognize the great things that have happened in metal since the 80's (although it's possible that these things have been allowed to happen because of the lack of scrutiny from the public eye, and a freedom from the economical pressures which govern the pop world)
Anyway, before I ramble of on another tangent...
When all they know is Metallica, it's hard to look beyond, without making an effort to understand metal, this is all they will know.