The No. 1 reason is the tens of millions of parents south of the border trying to decide to drag their kids across that border by any means available. Their goal is a better life for their kids and they believe that permanent residence in the US, even illegally obtained, furthers that goal. They are watching to see if DACA pays off for the current tranche of Dreamers. If it does, then the
next flood starts.
There is no fix until we stop giving prizes for illegal entry - no matter who was the instigator.