There's an explanation for that given in one of the Jedi Academy books (I think). Kessel is a planet with spice mines that is located near a cluster of black holes called the Maw (IIRC). Smugglers fleeing the authorities would fly around to the other side of the Maw to get away, and the faster your ship can go the closer you can get to the Maw without being pulled in. Your pursuers had to fly further away, taking a longer path while going slower, and you'd be able to outrun them and escape capture. This is called the Kessel Run.
So, the shorter the distance - the faster the ship. Any ship that can make the Kessel Run in less than 5 parsecs... well sir, that's one fast ship!!!!
