Aizen doesn't need to give Ichigo time to master the move. that's what the hyperbolic time-chamber is for. Aizne has one hour to spend not doing shit. he can practically go to a restaurant and wait for dessert, and it will be done.

maybe Aizen actually wants to make Ichigo into the key and not the city, maybe he wants to protect the city by killing the evil spirit king, or whatever silly Itachi plot shit thing.
how many chapters has it been since Aizen turned into the big bad?