Because people like AOC in congress won't vote yes to allow the funding. They want problems.
#10 | POSTED BY FISHPAW
They refused to fund more cages. These people don't need to be detained for a misdemeanor charge. It was not done under previous administrations, only those charged with felonies. Also asylum-seeking is not a crime, under international laws signed by the US.
For the most part, they have to enter the US in order to request asylum, so their entry is therefore not illegal. Detaining them at a port of entry (where it is legal for them to enter) is circumventing the law (what POTUS likes to refer to as "loopholes").
The US, by the way, is not "full". The average population density between the Mississippi River and California is ~10 people/sq. mi. www.census.gov