Because he could be charged with treason and the guards could have arrested him. Torygg was known to be a huge Empire supporter. Would you ask a guy to betray someone he's obviously loyal to if your life was at risk? He already spoke up at the last moot just enough to avoid being accused of treason. What more do you want?
And blasting Torygg and running him through wasn't an act of treason in the Empire's eyes? He's going to Solitude to piss off the Empire no matter what, why completely alienate the nords who were still Empire-inclined by killing the king?
Also... Arrest Ulfric? Do you realize how difficult that'd be? He knows the Disarm shout. And the order to arrest still would not protect the affront to Torygg's honor that Ulfric's challenge would inflict. That's why Torygg accepted the challenge in the first place.
So much needless bloodshed all because Ulfric wanted a go at the Empire more than he wanted a freed Skyrim. Sorry, I meant because the Thalmor have been goading Ulfric on in order to sow more strife amongst those against them and increase their odds of winnng Great War Two.
That would achieve nothing. Like I said, he already tried, and Torygg said nothing. He already took a big risk doing that. Anything more would have been stupid on Ulfric's part.
Achieve nothing? A woman who helped raise him from a babe said he had so much respect for Ulfric during the last moot for speaking his mind. Just like Ulfric had to speak in terms "just shy of treason", Torygg couldn't have just come out and said it either.