Here's the cliff notes version. After he was fired for not wanting to sing the songs the band wrote and then formed his solo band, Ozzy did everything in his power to trash his former band and his replacement Ronnie James Dio. There's an infamous BBC radio interview of Ozzy in 1980 saying that Sabbath is dead and the line up with Dio was a mockery, as well as on numerous occasions going out of his way to attack Ronnie for his stature and height. He even went so far as to hire a little person named John Edward Allen, named him Ronnie, and would do a mock hanging of him at the end of every concert. Yes...Ozzy was that insecure. In fact, Ozzy even did an entire live album of Black Sabbath material called Speak of the Devil, as a petty middle finger to Black Sabbath's Live Evil album.
Dio, however, wasn't gonna get pushed around by Ozzy.
Furthermore, after Ronnie left Sabbath, Sharon and Ozzy would leverage Ozzy's popularity with venues and concert promoters to nix Black Sabbath tour dates at risk of never getting Ozzy tour dates in the future. They even went as low as to calling venues the night of and saying "Black Sabbath will not be performing tonight" and cancelling gigs on them.
Now...here's the part where I would show more links...but...the problem is that due to link rot on the internet and Ozzy having his legal team get this stuff get pulled down (gee I wonder why), it's becoming harder and harder to find this stuff more easily than in say...the mid to late 2000s. Which was precisely before Ozzy sued Tony for a 50-50 split of the Black Sabbath trademark. None for Geezer and Bill though...because...reasons.
