Because:
1. They were probably pioneers of a genre at the time - despite possibily being surpassed by modern acts they are still considered the absolute Gods of a certain genre.
2. Mass appeal - follower/sheep thing, if someone is popular it's generaly subconciously taken for granted by the masses.
3. They have more money behind them meaning higher quality generally and better marketting etc etc. They probably have a history meaning they bring an authoritive stamp with their name, more likely to recieve investment blah blah.
4. Nostalgia.
5. With certain bands, ther's a 'getting into' factor. Metallica have it (didn't like them initially, then it clicked) Iron Maiden have it (Don't get it at all, postively reject it). This could simply boil down to the personality of band members, I guess. Muse probably have it. They are like pop-Radiohead at the moment.
6. Right place, right time. Or 'introduction band'. Tapping into a certain generation. For me it was Oasis, had zero interest in music before them. The music was simple enough to appeal, and they seemed to capture a certain spirit of the times. Plenty of radio play helped too, coupled with practically no exposure or anything beyond a general awareness of previous rock music on my behalf. It was new and exciting from my perspective. I remember the only other acts in the charts were stuff like Lightning Seeds and Crowded House, they just got blown out of the water really.
7. With Slayer, it's pretty obvious really. Two massive massive riffs/hooks in Angel of Death and Raining Blood just slaps you in the face with pumelling rythym that instantly have you worshipping them as absolute pure geniuses.
8. Lack of talent? Delivery plays as much a part as playing lot's of notes fast. You might dismiss something as crap and beneath you when years later it comes back and blows your head off. Taste is so subjective, can vary from being in a certain mood to being of a certain age.