Because the US Constitution says that border walls are unconstitutional…
A federal judge rules against the Trump administration’s attempts to build a wall along the southern border.
In their decision Friday, the U.S. District Court of Northern California added a number of states where construction of the wall will not be allowed. Those states include Arizona,New Mexico, and Texas.
The judge also solidified a previous order barring wall construction From may, which pertains to California. [sic]
This comes amid an ongoing battle for funding to build the wall, which the judge in this latest ruling called an attempt to bypass Congress. The president has since denounced the ruling as being against border security, and in favor of crime.
https://www.oann.com/9th-circuit-judge-rules-against-border-wall-construction-in-ariz-texas-n-m/