Because all it takes is one example to disprove something, but it takes an infinite amount of examples to prove something. For example: from experience you can expect that if you let go of a pencil it will fall, but you can't prove that the pencil will always fall. However, it's easy to disprove the contention that if let go of a pencil it will hover in mid-air and not fall - all it takes is one trial to disprove.