During the game we can choose between one faction, either Imperials or Stormcloaks. Why is there no way to remove the leader of the winner? There is the dark brotherhood and one of the quests is to eliminate the emperor, so it should be no problem to do the same with Ulfric or Tullius. One or all of the former Jarls could give the contract killing.
Another possiblity would be to kill both before the end of the war so that a peace between the two enemies coul be negotiated with a neutral leader.

The reason is that with my char I decided to support Stormcloaks but I am not happy with Ulfric. Somebody like Elisif the Fair seems to be a better choice.

Considering Elisif is the Jarl of the Imperial main city in Skyrim, I don't think she'd make a good replacement.

But she is highly esteemed by the ordinary people and also respected by the Jarls, Ulfric and Tullius ain't. It was only an example. My question was why I am not able to assassinate Ulfric or Tulius, wheraas I am able to kill the emperor.

You need both of them for the main quest.

For what, the temporary ceasefire? That is what I meant in the first post with the peace and maybe a neutral king/queen.

And if you want it, the option to kill the winner/both could be unlocked after the main quest.

As far as I know even her own thane only believes in the "leadership of general tuilius", not to mention other jarls.