As was said in the movie, the station was designed around a "direct large scale assault". They didn't anticipate "snub fighters" ever mounting an attack.
Tarkin and his lackeys believed their own hype about the Death Star being invulnerable, and scaring people into submission once word spread about Alderaan. It probably had a light complement of TIE's the same way Titanic didn't have enough lifeboats.
In Jedi, the DS II seems to have a lot more fighters protecting it.