Isn't having a bankai a requirement to being a captain?
There ars 3 ways to become a captain
1. Captain Proficiency Test: A test which requires the ability to perform Bankai. Nearly all Shinigami become captains using this method. At least three existing captains, including the Captain-Commander, have to witness the test.
2. Personal Recommendation: To have personal recommendations from at least six captains and approval from at least three of the remaining seven.
3. Trial by Combat: To defeat a captain one-on-one with at least 200 witnesses from the captain's division. This method is seen as being the exceedingly rarest and the least refined of the three. The method is commonly looked down upon by some captains as barbaric compared to the other refined methods. This method allows one to bypass having the mastery of Bankai or other possible abilities which a captain would otherwise be required to have knowledge of, for it allows no judgment from captains who would require those skills as a prerequisite to join their ranks. Though rare in other Divisions, the Eleventh Division has been set up to exclusively utilize this method, with each captain attaining their rank by killing the predecessor
Zaraki didn't even have shikai until this last arc