Why were people writing Fetterman off so bluntly near the end?

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People got so married to the idea that a midterm should go badly for the incumbent and, coupled with Republicans outperforming expectations the past couple cycles, people got nervous (or overconfident, depending on their perspective) to the point where they forgot to pay attention to what was actually happening. There really wasn't a point from Oz and Fetterman winning their respective nominations that Fetterman didn't look like the favorite. At his peak, Oz was an underdog within striking distance. But it's not hard to come up with bigger upsets recently than Oz winning would have been, so it's not that weird some people listened to their guts over the evidence in front of them.

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The debate probably helped Fetterman. If you're a normal person who's brain isn't poisoned beyond repair, you're not going to be mad that a guy who just had a stroke isn't giving the performance of his lifetime, it's going to make him come off as sympathetic. Oz, on the other hand, thoroughly embarrassed himself without the excuse of just being checked out of a hospital. A doctor mocking someone for having a health problem is a real bad look, plus he gifted Fetterman a free attack ad with that "an abortion is a decision between her, her doctor, and local political leaders" line, which helped remind voters that this was a weird nerd who appealed to nobody anywhere.

That being said, I doubt it had much of an impact whatsoever, because nobody watches Senate debates. The fact that Republicans hung their hat on it as the thing that would save them was honestly the biggest red flag Oz was toast - it's the kind of desperate clutching at straws argument you only come up with when you have nothing