It may seem odd that Renji never became a Captain after displaying his Bankai and fighting skills throughout Bleach, but there are valid reasons for it.
Bleach's Grand Fisher personifies the greedy ambition and arrogance of a Menos Grande while without actually being one, which is unusual among Hollows.
Even the elite Hashira don't take advantage of their position of strength, and they will never succumb to naivete or arrogance in the face of Muzan's Upper Moons.
Bleach's Quincy are back with a vengeance after a thousand years of slumber, and they're stealing Bankai to ensure victory. Fortunately, the Captains have a plan.