The reason is that while the attacking player sends over their workers to attack, the defending player is still constantly making workers. By the time that attackers get there the defending player should have more workers. He simply has to command all of them to fight the opponent's workers.
If the opponent doesn't react, you win because you kill all their workers.
If the opponent overreacts, you win because you are mining more than them (IIRC this is a problem with the insane ai).
If the opponent reacts poorly in other ways, you might be able to win by out microing them. E.g. SCVs (terran workers) consistently beat drones/probe (the other races workers) 1-1 without micro. Repairing can be an advantage. The offensive AI being better at stacking damage on a few units. Etc.
The defensive AI should learn to fix these problems and stop losing to worker rushes, but it could take awhile. In other words the defensive AI starts out as a lower ranked player.
PS. I'm not sure how serious a tournament this was, but here's an example of a real worker rush (not an early pool with workers attached): https://www.youtube.com/watch?v=_AtqgaJzJP4
Of course you lose all of that time it takes for your workers to drive across the map, so it's a pretty dicey strategy. A 50% win rate against the AI sounds about right.