Torygg was a King because the Empire made him king, for he was loyal to THE EMPIRE.
Torygg was made King by the Nordic Moot...which wasn't influenced by the Empire. Ulfric was there, he tried to become High King at the moot, he was overlooked. Torygg was chosen, much rage occured, Ulfric killed Torygg to become High King.
Sometimes I wonder if people ever actually look at the lore...

http://www.uesp.net/wiki/Lore:Torygg
http://www.uesp.net/wiki/Lore:Moot