The name was adopted from their minor league team that oddly enough only lasted 4 years. I guess from bad investing
The Williamsport Millionaires were a minor league baseball team based in Williamsport, Pennsylvania from 1906 to 1910. Many teams in this era never adopted formal nicknames and neither did the Millionaires. The Millionaire name was used by newspaper writers of the era in reference to the high salaries that the Williamsport owners were able to pay to the members of the team.[1