Everything changes in the playoffs. In basketball, superstars are guarded in completely different ways that leads to other opportunities for guys (who become "legends" because someone else double-teamed all night). In baseball, players are pitched in a completely different way. The weather is completely different in October than it is in June or July. Schedules and routines are thrown off by things like media availability and travel.
There are million things that go into all of this that the players have absolutely 0 control over -- our brains like stories, so we spin it into "this guy is clutch, this guy isn't."