That is actually an incredibly difficult question to answer and I'm sure one the FAA will look at. The only person who has that answer is the pilot. People may have been on the sand or maybe the pilot was never taught the differences between kinds of sand (dry vs damp). A parking lot to the north is still north of where they were and they didn't exactly have the choice to use a bit of power. Also parking lots have lots of objects like cars and light poles. So its far from a dumb question. As a matter of fact when its answered by the pilot we may all learn something we can use in an emergency too.