Well, there is two reasons for that. The first is that unlike Brawl (= port priority), the result screen of the game should not be ignored, because we have no reason to. So if the game says a win then it's a win, and if it says a lose then it's a lose. The only remaining question is when sudden death occurs.
There is two possible cases when a match is ended with a suicide move :
- Ganoncide always wins in the victory screen (so the sudden death case is irrelevant).
- EVERY OTHER suicide move either makes the initiator lose (in the victory screen) or go to sudden death. For Bowser it depends on the stage played on, for Kirby (if I recall correctly) it is which way he is facing, etc. But they never, never win.
So there is a few options from here :
- Making the initiator always wins, which can go against the victory screen (heck, a Villager can recover from a Bowsercide for example). Makes no sense at all and is a very bad grandfathered rule.
- Making the initiator wins in case of sudden death, or using the timeout clause. It means the results can be inconsistent for a same move (sometimes loss, sometimes wins/timeout) ; and also means that every existing player has to memorize all possible outcomes. For example, it means that every player has to know the result of a Bowsercide in every legal stage, including Omega forms (so 50+ cases) in case they face a Bowser in tournament. This option is not really "uncompetitive" per se, but it is a huge pain in the ass to learn that much hard data for a very low reward.
- Making the initiator lose in case of sudden death. It is a clean and simple rule ; but more importantly it provides a consistent result (across all stages and everything) much more inline with the results given by the game (which is, again, NEVER a win). That is my second reason for that rule.