Very simple. The league is currently lacking in players with the ability or the need to do so.
Lebron doesn't want to. Kobe's old and coming off injury. KD wants to be more like Lebron. Sabo has KD on his team. George's team doesn't really need it from him. There is no real reason why Melo shouldn't be dropping 50 every so often. Steph did it last year and I could see he or Klay getting hot and doing so again. Harden can do it.
The fall off of big men has something to do with it too. You look throughout history and generally the biggest, most efficient scorers were big guys. What big man in today's game is capable of dropping 50?
As far as winning and losing goes, sure teams may win individual games where a guy goes off for 50 but generally speaking if you team requires that kind of effort from you consistently they probably aren't that good. Only time we've ever seen a team succeed with one guy gunning is the 01 6ers and that proved to be a flash in the pan. For the most part teams like that barely make it and are first round fodder (see 05, 06 Lakers, Bulls early in Mike's career, early 2000 Magic).
Also

/

at people dismissing certain stats while praising another stat.