Why can't Wrath as a King fight alongside his brothers?

As the strength of the race the king's life 'matters above all others'. Because his life is so valuable to the race his life can't be risked. When he faught before he hadn't ascended so although he was technically the king he wasn't actively being king.

Because Wrath is now the "Blind King", with the lose of his sight he can no longer fight the lessers with his Brothers.

He and Beth don't have any children so who would take the trône if something happened to Wrath. Plus the brothers would be so busy making sure that there king was ok that they could get killed.

As king he has the responsility of the race and also has to deal with the glymera. There is no heir to the trône and he gave up fighting to be with beth.

Wrath is the last pure bred vampire and plus importantly, he is king. Wrath can help the vampires thrive and ultimately survive. Would any indispensable leader go 'trolling'?

Because he's blind and Beth doesn't let him in to fight.