It was raised in the maze. The Minotaur was the child of his wife so they just had to put it in as a baby. Also, there was a door to the labyrinth, so it could be shut. It is also easier to keep someone from getting through a door than to fight them normally. It also may be worth noting that part of the reason few could beat it is they were thrown into the labyrinth without weapons if I recall correctly (not to mention it knew it's way around).
Owrtho