He doesn't play the classical championship anymore because it's 6+ months of daily hard work to prepare for it and he's already won it 5 times.
"to the point of becoming bored"
Preparing for chess tournaments is always hard work. But the point is that he no longer has motivation to do that hard work to participate in the championship, since he's won it so many times and the result is never in doubt.
He said previously that he would defend his title again if his challenger was Alireza Firouzja. In other words there are certain things which would've caused him to be more interested /care more about competing, and thus motivated him to do that hard work. Make any sense?