Ozzy got the job because a local band needed a singer with a PA. The guy is almost completely untalented, literally one of the most overrated vocalists of all time. Tony Iommi himself said the reason their is no Sabbath live alum with Ozzy is because Ozzy was constantly out of tune. Meanwhile, Dio got the gig because a large, established band need a talented and professional singer. There is no comparison.
If Mickey Mouse was the first vocalist of Black Sabbath, everyone would talk about how Mickey is a great frontman, sang all these classic tunes like "War Ducks", "Iron Mouse", and "Mice Wear Boots", and we would all be wondering who is playing the next MouseFest. He's the Michael Anthony of that band.