Because its not "just as" prevalent in blue states. Quite the opposite actually. There are 16 states that do not allow exceptions and most of them are blue.
You singled out California as if its some exception in a failed gotcha moment.
Most states allow it with parental consent and adjudication.
Real question would be, why republicans want it lowered. Probably they dont even want a judge to he involved. Then they can marry children with no oversight. 😉