This is only my guess and not based on any fact, but I would gather for at least a few reasons:
1) To prevent a further advantage being extended to locals by giving them the ability to bowl multiple times, where most bowlers are traveling great distances and can only afford to once.
2) To prevent talented bowlers from regularly bowling and making a living in the brackets & pots
3) The tournament already runs for 5 months (requires booking an entire convention center for that time) and the squads are already full most of, if not the entire year
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