User blog comment:Xilinoc/Ch. 549 - The Stormbringer/@comment-1747976-20130917143559/@comment-11051098-20130918225258

Ittô Kasô is forbidden because the user has to use his own body as a catalyst to use the spell. And as we saw, even a captain as strong as Yamamoto lost an arm.

Regarding the pride, I remember reading something about him viewing the loss of his arm as a war injury, and he kept such injuries as a reminder of his battles. Maybe it was part pride and stubbornness, but it could also be a reminder of how he failed to defeat Aizen.