Defending titles is not easy. In the last twenty years, only Mick Doohan and Valentino Rossi have managed to win successive championships, despite both Jorge Lorenzo and Casey Stoner winning twice. Why is it so hard? A lot of reasons. Nothing motivates a rider, a team or a factory like losing.
Winning a championship requires a lot of hard work and talent, but also a smattering of luck, and at some point, luck runs out. Winning a title means always looking forward, eyes on the prize, while defending a title means looking back, at everyone out to get you. All these things combine to make winning the second title in a row much, much harder than winning the first one.