Because too many important people like politicians, Dr's, lawyers, cops, judges and their kids get them. If it were an automatic felony it would screw over way too many important people.I don’t know, so why?
In Canada - we don‘t have the misdemeanor/felony distinction (though we have something similar) we treat DUI (or impaired driving as we call it) seriously. It’s in the federal criminal code and applies equally across the country.
That's why it's stupid that a DUI and stealing a candy bar from a gas station are the same in terms of criminal offense.