Metal, like any other musical style, is functioning on popularity. As Zephyrus put it, usually a band becomes popular because of either them offering something new or being promoted properly. Extensive touring and multiple magazine appearances will get you more fans. There is no doubt in that.
What I find amusing is the irony that metalheads in general (as a social group let's say) are people who, at least on the musical level, shun pop culture and all things trendy and mainstream yet within their own subculture, they function the exact same way the rest of the world does. They listen to what everybody else is listening to. That is why, for example, you have a much higher chance of finding a metalhead with a Opeth shirt than one with an Astriaal one.
Not only that, within the metal culture there are the sub-groups. Those who prefer black metal or power metal, or doom, old-school thrash etc...
All in all the music that becomes the most popular is the one that's publicized, that gets more word of mouth. Also landmarks like Black Sabbath or Slayer are always going to be revered simply because they've been around long and they've been within the first to play the way they do.
Personally, I tend to be very picky in my music and for me, being told by someone that this or that band is awesome, is not enough for me to like it. I have to listen to a band to form my opinion.
Geralt of Rivia said:
then are the more famous and more prestigious bands really the best material metal has to offer as a genre?
No. I don't think so. Actually, for some, it tends to be the other way around. Popular bands can also be groups that offer a more accessible (diluted) version of the style they're playing. Just compare Metallica to Dark Angel (both popular I know but 12 year-old girls listening to M.I.A know who Metallica is...)I find that younger metalheads will generally start with the widely-known universal bands everybody seem to like and slowly start discovering more underground bands that play music in a way they like even more...My introduction to metal was In Flames (back in the Colony days) and since then I have discovered numerous bands of different genres and the one I like the most are not always, even rarely, the most popular ones. Heck, I don't even listen to In Flames anymore.