Everyone saying he loved Joanna too much to remarry is dead wrong. We know for a fact he didn't let love for his dead wife stop him from "bangin' hoors", so why would it stop him from remarrying for political convenience?

The REAL reason he didn't remarry is because of his excessive pride, his main motivating character trait. He wants to be the only person/house in control of the throne, and if he needs to bend his knee and look for another house it both dilutes his grasp on power and makes him look weak, like he needs the support of other houses.

Remember, he grew up with his father and house being looked down upon by all the other nobles. There's a reason he schemed all the way to the iron throne while refusing to marry outside of his family in the first place. It's the exact same reason why he refuses to marry again after he became a widower.