Because of his poor debate performance.
I remember how much sh*t I got for saying the debate was a wash. We all knew Fetterman wasn't going to perform, but Oz just came off as a desperate dick. That and him endorsing people like Doug Mastriano having a seat at the table was just self-immolating.
People went into the debate with two preconceived negatives. Fetterman was a mealy-mouthed candidate clearly still recovering from his stroke. Oz was a self-serving, smarmy douchebag who would say anything and everything to get elected. Oz distanced himself from his campaign mocking Fetterman's stroke while mocking Fetterman's stroke on the debate stage. He told a group of Republicans he would ban abortion in any circumstance, he told everyone he would let "local political leaders" decide, he told others that he didn't support a ban in case of rape and incest, and he was unabashedly pro-choice outside of politics.
And nearly everything that came out of his mouth was a carefully-rehearsed, hyper-polished talking point with "My opponent, John Fetterman" rolled into it. For all the talk about Fetterman reinforcing his negatives, people overlooked Oz reinforcing his own.