Why does Oklahoma have 3.2 alcohol beer?

It started with prohibition. During prohibition, nationwide 3.2 beer was considered "non-alcoholic" and was found everywhere. When prohibition was lifted some states (mostly in The Bible belt) decided to keep it and regulate 3.2 beer differently than higher alcohol content beer, which is then treated the same as liquor. If you want good beer in Oklahoma, go to the liquor store and buy it.