Amajor reason why the majority of supreme court justices have had political experience prior to appointment to the court is that

The answer is "Presidents seek to place individuals on the Court whose policy views are similar to their own".

Political contemplations ordinarily assume a critical part in Supreme Court arrangements. It is regularly accepted, for instance, that Presidents will be slanted to choose a candidate whose political or ideological perspectives seem perfect with their own. The political idea of the arrangement procedure turns out to be particularly obvious when a President presents a chosen one with dubious perspectives, there are sharp factional or ideological contrasts between the President and the Senate, or the result of essential established issues under the steady gaze of the Court supposedly is in question.