
Aizen kept these four locked in Hueco Mundo for a reason.
Even Aizen (with the Hogyuko, mind you) still didnt wanna deal with these four. Especially since who know’s what type of weird/cooky contingency plans Mayuri has set up to deal with Kyoka Suigetsu and even the Hogyuko. Aizen couldn’t risk dealing with Yama and Kisuke along with Mayuri, Unohana, and Zaraki. Even tho Byakuya is the least of a threat to Aizen out of these 4, I do believe Byakuya would’ve fought better than the vizards/other captains already in Karukura town.