There is no good reason. It's really a loophole in the law. An understandable one, because cancelling a trade is not legally the same as making a trade, even if financially it's the same. And one that's small enough not to have been worth closing. You won't get away with cancelling your plan every time sales data is bad, you should wait for the once-in-several-lifetimes opportunity when you get game-changing news and you were just about to sell a large quantity of stock.

You can come up with a bigger conspiracy theory than this, but the more boring explanation looks true.