Coming late to this discussion, but I think everyone is forgetting some fundamental ideas:
* Metallica had such an influence on the music scene in the 80's. We may all sneer at them now, but we can't forget that four album ride from Kill to Justice in which they were metal personified. From a metal world, they are deserving of being inducted, even though the many other bands discussed above are as well.
* We hang out on metal boards, so it is easy to think that the world of music is one of metal. In mainstream society, Judas Priest is an afterthought that has had a few radio songs (Midnight, Another Thing, maybe Law). Alice Cooper is one of those slightly underground rockers that has also been around for decades. Everyone knows him, but I doubt most people could name more than three of his songs. With the Black Album, Metallica went from one of the top underground metal bands to one of the top *bands* in the world. So, to the mainstream RNR world Metallica is a shoe-in.
* Finally, as has been pointed out, it's also about things like name recognition, record sales, and saving enough big name inductees so they have a few to bring in every year. If they started out bringing in all of the big names in the first year, they'd be down to, what, George Thorogood and Wham! by now. The RNR HOF is a production and a marketing venture, and you should treat it as such.
Steve in Philly