The information is hardly scarce, by my standards (I can find more information today than I ever could back in 1992). There's information publically available that supercedes pretty much everything "outdated."
It's also fairly well-known that after about 1994 or so, people started giving one another SNES development manuals. Sure, it's hundreds of pages to photocopy, hole-punch and mail out, but people were doing it. If you know the right people, you can get this, and it basically has everything you need to know (hell, it's all that even the professional developers had).
The reason why SNES development isn't such a big deal anymore is because that era is over. The SNES development scene is long dead; if you can still find archives of the snesdev and famidev mailing lists, take a look at the dates in each post. Everyone did demos and trainers, and programmers kept doing more and more with the system. Everyone was doing it because they wanted to, and because it was fun -- emulators were not in ANYONE'S mind, because the idea was silly (why emulate when you can use the real thing?). Now with the coming of emulators, the stigma has essentially died with it.
"So if the SNES is newer, then why is everyone still making NES emulators?"
Because for some fucking reason, people think the NES is the MOST RADDEST AWESOMEST SYSTEM EVAR, DOOD! I REMEMBER WHEN I GOT MY NES, I WAS 4 YEARS OLD!!! MAN IT WAS THE SHIT, HEY WANNA SMOKE THE REEFER? MAN THE NES IS SO COOL!! I WANNA HANG OUT IN #NESDEV AND BE AN ELITE COOL DUDE LIKE THESE OTHER ELITE COOL DUDES, DUDE!!!
That's essentially the mentality today, which is why you can find probably over 100 different NES emulators. Everyone thinks the NES is "easy" because it's old, 8-bit, and cheesy. Bzzzzzzzt. Wrong. I consider the NES to be the most technically complex console I personally have ever worked on. Consoles like the Atari 2600 scare the living shit out of me (counting cycles to put pixels on a screen -- ONLY! -- isn't my idea of fun).
The SNES was logical and very easy to deal with; everything NES developers dreamed of was in the SNES for the most part. H/V counters are built-in, DMA is present, there's a seperate CPU for audio (8 digital channels, in stereo), 128KBytes of RAM, 256 colours, 16-bit registers with a changable zero (direct) page, yadda yadda yadda...
I think you'd have an easier time programming for the SNES than you would the NES. Just happens to be that Memblers and I disagree on the matter -- no big deal, everyone has their experiences.