Why do some airports have class E airspace to the surface only at certain times?
The class E airspace protects IFR traffic departing and approaching the airport from incursions by VFR aircraft.

visibility and cloud clearance requirements for Class E are more strict compared to Class G (which is what the airspace reverts to after-hours).

3 statue mile visibility, 500 below, 1000 above, and 2000' horizontal from clouds for Class E

Compare that to 1 sm / clear of clouds for Class G

You could see that someone pushing the limits of VFR, scud-running to/from the airport might pose a danger to an IFR aircraft on approach to the airport and might pop out of the clouds and get a surprise.

As to the choice of hours, that's likely an FAA decision based on traffic volumes.

Normally you see this at airports that serve rural locations with scheduled passenger service throughout the day (and possibly later at night as well).  Pellston, MI is another airport that comes to mind with a Class E surface area, but it is 24 hours a day.  I think Delta serves Pellston with 1 or 2 flights a day from Detroit.