Have no idea the reason, though to me, a flight number is simply that, and no real confusion if it changes. It just changes, just like a departure time does on occasion.

I'll admit, it was convenient when I knew that the ORD-YYZ and return was always numbered 1100 - 1115 (1113 was not used, of course), in order of departure time, with even numbers north, and odd south. And that when I frequently flew the same flight departing at the same time, it was always the same number. But really, in the scheme of things, what's the big deal? The flight number really doesn't change anything.