Biden clearly had problems earlier, but 1) some people didn't want to jump the gun and pass a judgement that could be seen as being premature and politically motivated, 2) Biden had his physical this month so his mental state is particularly relevant now, and 3) Brit Hume isn't on TV that much now, he probably wasn't asked about this earlier, and on those other occasion she may have had other things to say that were more important at the time.
The swampers like Hume didn’t want to undermine Biden while Trump and MAGA were on the ballot. Now it’s all about the midterms and electing assistant Democrats. So Hume is just fine with taking a few shots at Biden.