Several reasons:
1: The tune and the vocals need to fit. It won't matter if it has the best guitar/bass/drums in the world if the singer screws up. This also means that songs are written to fit the voice of the singer, making him more important.
2: The most impressive instrumentals often don't sound as good as simple clean tunes that are rather simple to play. Oasis' "Wonderwall" is a good example. There are exceptions of course. You have pure instrumentals that are awesome ("empire of the sun" by Edenbridge is a good example of awesome guitarplay)
3: The vocalist is the face of the band.
4: Anyone who brings up Metallica as an example should be ashamed, as they're greedy sellouts whose musical hearts stopped beating a long time ago.
5: In most other genres, there's a solo vocalist, this gives a mentality that affects how you view bands.
6: Most of the good old times when talent and awesomeness gave you fame is over. Rock isn't dead, but it sure ain't moving much these days. (Personally, I think it's just biding it's time for when it makes its glorious comeback)
7: Most people are braindead in all fields they don't give much interest to, so unless the person really loves music, he/she won't bother knowing much about them anyway.
8: Nightwish is a peculiar case, because the vocalist that replaced the first hasn't got the same opera voice, and although the first wasn't an active songwriter, it really shows that changes happen when the vocalist changes, for better or worse.