Because the movement needs to pressure Kamala into actually working for a ceasefire, which is not currently what's happening. Saying that Kamala is working tirelessly toward a ceasefire invites the question of why people were outside protesting the DNC. I mean, I've been seeing liberals use this same line all over the place as an attack on the protesters: "what more do they want, Kamala/Biden are already working on a ceasefire". It isn't true and anyone actually wanting a ceasefire shouldn't say it.
That seems like two distinct issues you're raising. One is whether the Biden administration has been somehow working on getting a ceasefire. It seems pretty clear, to me at least, that they have been doing so. There is reasonable disagreement on whether the efforts have been sufficient. Certainly, they haven't been successful so far, so that alone is a good argument that they haven't done enough, but I don't think its reasonable to say that they haven't tried. They've been involved in active negotiations for months, imposed sanctions, made speeches, issued vague threats, etc.
The second, is the claim that Kamala Harris was somehow personally involved with the ceasefire efforts, advocacy, negotiations, etc. That is a slippery one, because Harris and her campaign have certainly made public statements supporting and advocating a ceasefire, and she is part of the Biden administration, which has done the same. What role, if any, she personally played in hands-on negotiations, policy crafting, etc., seems unknown or at least less known, being generous. The less generous conclusion is that she didn't do jack-squat other than give it some domestic lip-service.
Additionally, within that second claim, there is an additional nuance, which is the "working tirelessly" phrase. Regardless of what actions, if any, that Harris took in furtherance of, or advocacy of a ceasefire, I would agree that the notion that she was "working tirelessly" for it, is laying it on thick at best, and just an outright lie at worst.
I don't think AOC's position was based on certainty that Biden would remain in the race. She didn't need to be certain of that - either way she would be increasing her credibility in the party by backing Biden until either the election or until he himself made the decision to drop out.
AOC also endorsed Harris full-throatedly on Twitter like shortly after Biden did so I think that was all that was needed to establish she was cool with it.
I don't know if a Twitter statement was enough. A primetime speech at the DNC , a nationally televised event was better, but even that may not be enough. I don't think we've seen the last of measures like this from AOC. She is trying to move up in the party.
Incidentally, AOC facetimed in to the sit-in and tweeted last night that the DNC should have a Palestinian speaker so she's clearly still trying to thread the needle here instead of just throwing in her lot with AIPAC.
"Threading the needle" I think is a perfect description of what AOC is trying to do, on numerous levels.