They changed it because players were able to breed unlimited amount of good offspring's (5x31, 6x31) with the same 2 parents which will eventually crash the market. So even if it did take 2 weeks to hatch an egg, that wouldn't fix the problem in hand which was finding a way to stop people from creating unlimited amount of good offspring with little to no effort. This is why they changed it to the current system, exchange both parents for the egg. The only difference is that you have more control by using Braces to pass on IVs, Ability Pills, and you also have the choice to choose between male and female.