Really tons more influential than Maiden? You seem to consider them "niche" because almost every metal band after them was inspired in some way. Their early material could be compared to other NWoBHM bands of the time but they were in a league of their own. Come 1982 no one had ever heard anything like Number of the Beast, they defined modern heavy metal.No they're not. They're Niche, and as you've said, more niche than iron maiden. The only speed metal band that will ever be in the rock and roll hall of fame is metallica. Slayer inspired a generation of metalheads, true, but this is the rock and roll hall of fame, not the metal hall of fame. Slayer won't get in because only a small number of people get in every year, and there are much more deserving and more influential acts than Slayer (and Maiden, for that matter)
Not liking them is fine (although I have the feeling you haven't looked much past their handful of hit singles) but saying they aren't important and "niche" is downright ignorant. They put metal on the international spectrum and really added a new life to it that no one had herd before.
Slayer influenced a much more underground genre but I would still say they deserve a place but most likely at a later date once bands like Iron Maiden and Judas Priest are accepted.