Bokuto is the only captain in the whole anime that doesn't wear number 1 jersey. Some think it's because he get attached to his jersey number and didn't want to change it when he became captain but I think it's more because of his insecurities.
He doesn't want to get seen as the captain when the other teams take a look at them, because he doesn't think he is strong enough to represent his team and he loves his team so doesn't want them to get mistaken as weak because of him.
Bokuto knows he acts too childish sometimes and his team mates are the ones consoling him or being the pillar most of the time while he should be the one to do it.
Bokuto is so proud to be the captain but he is scared of being the captain. He isn't good enough in his eyes and to wear the number 1 jersey being the captain is not enough.