I wonder if its because:
1. Just a buncha crackas died their.
2. Fear that Abe Lincoln’s ghost will kick his ass.
3. No 6 star resorts for his family and entourage to spend a week at.
4. Conflicted with tee time.
Probably because God is named in the address.
There was a great article in PennLive..
http://blog.pennlive.com/gettysburg-150/2013/10/obama_chickens_out_skips_getty.html