Making-up the Law after an Event

The Supreme Court Judges have made up the law after Parliament was Prorogued.  Not only that they decided that there was no prorogation after all, although the PM believed that what he did was lawful.  This is the same as inventing a law that introduces hanging after a murder has been committed.  If that is what the Judges have done, why didn't they anticipate the situation before?  These people are not fit for purpose.  How can the Judges say that the PM's decision to prorogue Parliament was unlawful when there was no law to prevent that decision?