Why did Aizen never use Bankai?

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Tags

1. #release
2. #without
3. #realizing
4. #since
5. #lieutenant
6. #bankai
7. #should
8. #might
9. #shikai
10. #aizen
11. #fetched
12. #ability
13. #become
14. #captain
15. #anyone
16. #think