There's no information posted about why these particular parameters were used in creating the new bill. But you have to have a cutoff at some point, or else every single person who owns a home in the U.S. would qualify for the credit.
My guess is that 5 years would mean there's a better chance that the person who's looking to sell their home would have some equity in the home. Also it's more likely that they may be ready to move than somebody who had just bought a couple of years ago. One of the reasons for the tax credit is to reduce the number of houses on the market, not create a huge influx of new listings.