Showing Respect
This theory seems to be popular, at least with regard to the British population. In fact, according to a survey that StatistaCharts conducted for The Week, nearly 50% of the British public want to see William accede to the throne instead.
The Proceedings
Allegedly, Diana had mixed feelings about Charles becoming the monarch. This should not come as a big surprise as their divorce did not end really end well. In 1995, she addressed this when she made an appearance on Panorama.