The premise of the overall Trump approach was that people who feared for their lives in their home country because of persecution don’t necessarily need to come to the United States to escape. It should be enough for them to go to another country in the region, or if they are indeed applying for US asylum, to stay in Mexico while doing so.
Allowing them into the United States, with no reliable internal enforcement mechanism, constitutes an end-run around our immigration system. Because migrants, like anyone else, respond to incentives, the more who are allowed in, the more will come. And, since our resources aren’t infinite, if enough families show up at the border, it inevitably overtaxes our personnel and facilities.
Even if Biden has different priorities, it makes no sense to create a willy-nilly rush at the border before a supposedly better system is in place (whatever that might be).