It was more than humility. John was smart.
The church was in bad shape in the 14th and 15th centuries (read Thomas Costain's history of England and Barbara Tuchman's "A Distant Mirror" for details). Becoming a bishop risked being co-opted by the corruption in Rome, and a genuinely holy man would have a hard time maintaining his sanctity either in Rome or playing politics in a foreign city. The smart ones eschewed ambition and worked as mendicant friars or holed up in a monastery. It was easier to save one's soul that way.
Even the great Francis of Assisi had his problems with the politicians of the Papal Court.
BTTT on the Optional Memorial of St. John Capistrano, October 23, 2006!