Most likely because he’s not feeling well, which can have a lot of causes.
It could be that he is tired because the temperature in his enclosure is not high enough for him to be active. too low temperatures can be a cause of dragons sleeping all the time.
He could have a deficiency of a certain vitamine, which can cause him to sleep/be tired a lot. This itself can have multiple causes:
Your
uvb light may be broken. Keep in mind a lot of uvb lights only work for about 6 months before they need to be replaced. Even with a good
uvb bulb, try using calcium supplements once in a while to support his health.
A lack of diversity in food, make sure he gets insects as well as his greens. Make sure the fruits and vegetables you eat are safe, and try feeding him different fruits/vegetables for a varied diet.
If you feel none of these things can be the cause of his laziness, all i can do is advise a trip to the vet