Minos imprisoned Daedalus in the labyrinth as punishment for helping Queen Pasiphae (King Minos's wife) to get close to the king's prized white bull and conceive the Minotaur (the half-human half bull monster). 

Daedalus was to be sentenced to death was imprisoned for life instead after suggesting the creation of the labyrinth to contain the minotaur and ways to make the minotaur useful to the king to gain back his reputation and be feared instead of being the laughingstock of Crete. 
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Because Minos knew that Daedalus help her daughter and the as us escape Crete.
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