Sump Pump Minimum Basin Height
Two things:

It allows room for the inlet(s) to the sump pit.  If the pit were too shallow the water might backup into the inlet pipe before the pump switches on.
It gives the pump a bit of &quot;buffer&quot; when you have sudden surges of inlet water.  If you didn't have that, it might overflow before the pump turns on and gets a flow going.

Also, check that your outlet pipe has enough capacity.  That might be the real problem instead of the pump.  You can only force so much flow through a small pipe.