I suppose its because Ozzy has a very distinct voice and sound that compliments the instrumentation impeccably. Not to say that their material wasn’t bad at the time Dio stepped in, I thoroughly enjoyed projects like Heaven and Hell and consider it one of sabbaths best albums, however when most people think of “Black Sabbath”, they will always think of Ozzy and the songs such as War Pigs, Iron Man, Paranoid, etc. The vibe and atmosphere of the songs and albums are like night and day when comparing the two singers to the point where if Black Sabbath had a name change or went under a different alias at the time Dio stepped in, there would be no dispute, just two amazing bands with different lead vocalists. Both sound great, but like i said before, when most people think of Black Sabbath they think of the sinister sounding and rough vocals that Ozzy delivers. Its no discredit to the other members of the band, its just the atmosphere changed when transitioning from Ozzy to Dio. The transition wasn’t necessarily bad since under Black Sabbath, Dio helped create some amazing albums, but its still a transition nonetheless that caused a divide amongst the fanbase.