What would cause my barely used green bearing to do this?

I never claimed a ball wheel bearing is as strong as a tapered roller wheel bearing.

Most people will never know the difference when used in a daily driver as millions of Ford, Toyota and Mopar rears came with them stock.

As far as what is "wrong" with tapered roller bearings, I can almost guarantee the OP can't install them without mock-up and careful measuring/machining. A tape measure rarely cuts it for a custom width assembly.