Because every other sport only has a three step podium? There's no real reason to have five, other than to add additional hype when two more riders get on the "podium".
The five place podium started in Cairns Australia 1994 when a young Cadel Evans (now retired Tour de France winning road racer) finished 5th in the elite XC World Cup as a junior.
I like it its good for sponsors, riders and the fans.
I like it its good for sponsors, riders and the fans.