Using the info in the Kathryn's Report,
http://www.kathrynsreport.com/2014/04/planes-crash-near-san-pablo-bay-and-one.html I used SkyVector to show the direct route from KHAF to CA20, did a quick screen scrape and edited it to insert a red marker where the collision reportedly occurred. See attachment.
Obviously they didn't take the straight route home through Class B but probably were circling under the shelf around the north.
CA20 is 4000 feet long and 100 feet wide. It also has 1200 ft structural overrun on the north and 2200 ft compacted dirt overrun to the south.
So their destination airport was comfortably long and wide and being a private field, no other traffic would presumably be in the way or involve controller intervention. Feel free to browse the nearby airports, but I could not find any airports that seem to have all those advantages going for them. Besides, it takes time in the cockpit to figure out alternates - sometimes it is easier to complete a flight to a known location than inject another variable that is as likely to add to the danger as subtract.
As most have probably experienced, landing at one's home field presents more known quantities than landing at any other field.
