His voice works for some people (apparently a lot of them) and I must admit there are moments where he hits this kind of blues-y undertone that is OK.
I think it's just that Ozzy's voice defined heavy metal. I don't think I love Ozzy's voice, but I don't cringe and turn away from him, at least on the classic tracks. It's as part of the roots of metal for me as Julius Caesar is part of the fall of the Roman Republic. It simply
is. Ozzy opened metal up to all sorts of vocal experimentation, simply by being different to what was being produced at the same time.
In 1969, the biggest songs in the world were Get Back by the Beatles, Honky Tonk Women by The Rolling Stones, Sugar Sugar by The Archies, and Suspicious Minds by Elvis Presley. It's like they picked Ozzy deliberately as a reaction to that, to get wildly away from the sound of popular rock and roll. Much like many people appreciate the early black metal days, with the low-fi production and reactionary sounds vis a vis the traditional metal scene, this must have been a similar break. The production on the first Black Sabbath album wasn't good, the guitar tone is incredibly divergent to the guitar tone used by McCartney, Lennon, and Harrison, the use of bass far more riveting and dynamic, and the voice, yeah, it's sonically different and wails all over the place.
Like, whether or not you like Ozzy's voice, he's part of that break, that huge bold push that created an entire genre of music, and vocal experimentation and the achievement of new forms of extreme? That's part of it too, and whether or not you like Ozzy for it you can thank him for the evolution of non-traditional vocals in metal.