The thing is, the Democrats might be against the wall and the kiddie concentration camps, but they have bought wholesale into the "we must militarize the border" dynamic that has existed since 9/11.
Of course, 9/11 was an intelligence failure, not a border control one (Mohammed Atta aside...he should have been flagged for sharing a name with a Palestinian terrorist) and none of the terrorist attacks since then have had anything to do with border control in any significant fashion, but the Democrats believe if they show they are tough on border security, Republican voters who think Pelosi is a Communist sleeper agent in the US to install sharia law, will find the party more appealing. As you can see, this strategy is working out great (you'll note Republicans never need to "appeal to moderates", even while the President is tweeting white supremacist propaganda and trying to get a Congressman assassinated by proxy).
So of course they voted against AoC's measures, because it would more broadly limit their ability to militarize the border.