The people who played the original game, if they were age 5-20 at the time, are now 36-51. At that age, time for exploring and developing hacks is harder to come by than in one's teens and 20s, if family is part of the picture.
Truth be told, it's difficult to design good levels, good AI, progression of difficultly, etc., even for people who do it professionally, let alone hobbyists.
I have one NG mod that has been in the works for 9 years now, and another that reverses the entire game (all levels mirrored) that has been nearly finished for a year and a half. All I have to do is write an array of coordinates where ladders can be climbed, since the game only supports ladders with Ryu facing to the right. It might take a couple hours and then be done, after many more hours that already went into it. But I have to slap my own hand every time I reach for it, because it gets me no closer to financial freedom, personal development or other lifelong goals I'm working on now. Wouldn't be surprised if other people are in that boat with me.
NG2 and NG3 are not nearly as good in my opinion. The physics just aren't right - too floaty. I also consider wall jumping essential to the ninja feel, and they removed it. The music in the first one is better, the cutscenes, the sounds, the powerups. NG3 is especially mean with enemies knocking you out of the air to your death, and the US version is brutal with limited continues. And its sci-fi theme is incongruous crap.
I agree with others that Deadpool is a very impressive hack that fixes the glaring flaws of the original (most notably, the awful boss fights and hitboxes), while adding loads of new gameplay. I suspect we will not see a hack of its depth again, for any NES era action game. People are too old, it's too much work, and longevity seems to favor RPGs, Mario, Mega Man and Castlevania.