Why does my dog not eat out of his bowl?

There could be a few reasons.

If you're indicating the dog will not eat at all, it could have dental/mouth pain issues or some other illness (a vet visit can help rule these two out), or be stressed or nervous, or picky/finicky about the food option.

If you're indicating the dog will eat from your hand or the floor, but just not the bowl, then there might be something about the bowl set up (uncomfortable height, if it's on a stand- moves around/makes noise, scent of another dog on bowl, or made from porous material such as plastic).