I believe that the Persians didn't care for the small land of Greece. Greece was poor and small. Invading Greece and maintaining it would probably have cost quite a lot more than what small profit it actually had. I think Xerxes was foolishly trying to set an example of how people who offended his empire would have been treated and failed, obviously.  Also no Persian ever thought that the Greeks would form a threat to the Persian people, or even be united and strong enough to even form a military unit.  

Conclusively, I suggest the Persians failed to take Greece because they did not wish to conquer it. The land was poor and rough, unsuitable for farming. It was far from the capitals and hard to control, should something happen in Greece it would take at least two to four days for a Chapar to reach the king in Susa or Persepolis. Furthermore, the people of the land were easy to revolt and demanding democracy and this was not to the liking of the monarchic Persian empire, although they allowed the Ionians to have their own popular governments, the Ionians were easier to control than the Hellenes in Greece. Thus the Greeks were best left to themselves.