What would stop him from getting a Bankai? I mean he already knows his Zanpakuto's name, would it be the materialization that wouldn't work?
No it's not that. It's the fact that there's, literally, no point in Kubo giving Zaraki a Bankai.
We just found out in his training with Unohana that he was
NEVER at full strength. He had mental limits on his power. Zaraki defeated the previous Kenpachi, became a Captain, killed Nnoitra and this was all while he was mentally limiting his strength. He even fought toe to toe with Unohana and gave her a run for her money when he was a kid.
A KID. A CHILD.
During his training with Unohana he got back to his prime, defeated her (the original Kenpachi), and learned his zanpaktou's name. I don't think people realize how much of a power boost this already is.
Imagine Zaraki:
- At full power (no mental restrictions)
- No eye patch
- Two hands
- Communication with zanpaktou
- Screaming at the top of his lungs
Add to this the fact that having a Bankai would ruin Zaraki's character. Ever since he was introduced, he was special because he was the one Shinigami in all of Soul Society's history to become a Captain without a Bankai.
It's what makes him unique.
Zaraki isn't getting a Bankai because:
- He's already strong enough
- It would ruin his character
- The Quincys would take it