https://en.m.wikipedia.org/wiki/Designated_hitter Pitchers are typically not good at hitting. The AL choose to adopt it since they thought it would help offense after a pitching dominated era. The NL didn’t adopt it.
In the past, the AL and NL were separate organizations, with their own management. Nowadays everything is run by the MLB main office and it's inevitable that the DH becomes universal at some point, but it has to be a part of a new negotiated CBA between MLB and the Players Association.