Well, whenever he used the Sharingan and the katon jutsus and such, the result ended up the same as if he only used Chidori: failure. He probably thinks that if he can't kill someone using his regular jutsus, then he might as well finish them off with the Chidori. Sasuke hasn't gotten weaker; it's just that he's only using one move rather than all the stuff he had in the past. Since he uses only one move and fails, it looks like he became weaker.