The NHL has been working around its overtime format. It used to be a 5 minutes 5 on 5 frame. It then got changed to 4 on 4 for a while and latter to 3 on 3 for the regular season. But watching the playoffs, I noticed that overtime is still 5 on 5. How come the number of players in overtime is different between playoffs and the regular season?
The NHL regular season overtime format has undergone quite a few changes over the years. The first season with sudden death overtimes was 1983-84. The whole purpose of including them was that people simply didn't like so many games ending in ties. Along with changes to the way points are given to teams in the standings, there have been experiments with adding shootouts and with reducing the number of players on the ice in overtime (so that it's now 3-on-3).
In doing this, the NHL has ended up eliminating ties, added a little extra excitement/drama to the end of games, and to a certain extent quickened the games up.
Other than maybe tradition, the reason that postseason games have a different format is that the regular season format is (for lack of a better word) gimmicky--just trying to get a quick, exciting result for the game. In the playoffs, more weight is given to letting teams play at full strength with all their players in a normal setting. In other words, they continue to play the way hockey was intended to be played and the way that exhibits that one is really the better all-around team. Also, an extended playoff game, perhaps reaching two or more overtimes, can be pretty gripping to viewers because the stakes are high and at any moment everything could be determined--but we really have no idea when. Compare that to a regular season game that has a lot less importance; if it had the playoff overtime format an average observer might just stop caring at some point--it starts to drag on, and the experience wouldn't be as unique.
Yes, the playoff overtime format is different from the regular season - in fact, almost completely different. As you correctly point out, regular season overtime is five minutes with a number of players that has varied through the years and then, in recent years anyway, followed by a shoot out. Playoff overtime is untimed and has always been five-on-five - it's just like a continuation of normal time, except that it's sudden death.