Bathory's most significant contribution to the development of black metal was in creating mystery via lying about one's influences and obscuring one's shitty derivative music with bad production. His first two albums are total jokes, worthless speed/thrash just one step above Thrash Queen's Manslayer, and while he'd improve significantly with Blood Fire Death, there's still so little on that album that Slayer and Manowar hadn't already done. I guess Under the Sign... might be his most interesting and all-around important album, but I very rarely feel the urge to listen to it, so I'll concede anything involving that one album alone. Doesn't make up for everything else. He was a hack with the fortune of having a father willing to fund his silly musical misadventures.
tbh I wouldn't cite lack of variation as a reason to why it's a largely failed sub-genre. If anything, it's so weakly defined that you can have some of the most deviant and un-metal albums still placed under its banner. Mayhem sounds nothing like Bone Awl who sound nothing like Drudkh who sound nothing like Nokturnal Mortem who sound nothing like Mortuary Drape.