Why did jesus clear the temple of the money changers?

Matthew 21:12-14 And Jesus went into the temple of God, and cast out all of them that sold and bought in the temple, and overthrew the tables of the money changers, and the seats of them that sold doves,
   And said unto them, It is written, My house shall be called the house of prayer; but ye have made it a den of thieves.
  And the blind and the lame came to him in the temple; and he healed them.

  Jesus cleared the temple of money changers because the temple was the house of God, a holy, pure place, and the money changers were being dishonest and cheating people.