To boost jersey sales and raise his image to iconic proportions.
Kobe Bryant is a very marketing savvy professional athlete. He understands the power that he wields being one of the world's top athletes. He founded and runs an advertising agency for crying out loud.
Kobe Bryant has an unusual level of self-awareness for a professional athlete. Changing numbers creates a an entirely new demand for people to rush out and get 'the new Kobe jersey.' His jersey has remained near the top of NBA jersey sales for his entire career. He has the talent alone to reach that level of popularity, but the way he presents himself and his personal brand has elevated him a rarified air as a global sports icon. This was not by accident.
It came from Kobe controlling a number of elements that made the world view him as more than just another talented basketball player. It came from how he speaks to the media. How he is viewed in commercials. How he presents his signature shoe. All the way down to the timing he chooses to refresh his jersey number. It's all part of a holistic plan to control his image and boost his value.