He wanted the Galactic Empire wholeheartedly and fully supported the Death Star unlike Vader who derided it as a technological terror. Furthermore he ordered the death of his beloved sister as well as his best friend without batting an eye. He had the same ideals as Darth Sidious! He was a true Sith while Kylo Ren, Darth Vader and Darth Maul were pretty much failures!
Sidious made the arrogant mistake of seeking someone who had attachments just because he was the Chosen One while Dooku was the perfect Sith!
If Sidious had a brain he would have immediately realized that the Chosen One(who is supposed to destroy the Sith) should be on the “Kill immediately!” list and focused on Darth Tyrannus while having him desperately seek a means to cheat death even if he had to rip the knowledge out of the Nightsisters!