Even if he ran, I doubt he would hardly be able to win, the last time in 2016, he won by a very small margin, like only swinging some key states by mere few thousand votes. And then his four year term, specially at the ending was so bad, in which he, apparently supported violence and probably the only president who was so mad and looked selfish when he lost the election without acknowledgement of the result and meeting other formalities. I doubt anyone will trust him again.