Each VC game has to be tested thoroughly to ensure that it hasn't any "new glitches" in the emulators they use, and it has to be rated again by ESRB, PEGI etc. Then they have to actually get the rights to third party VC games, that takes time.
And then they can't just release every game on one day because every game but Super Mario Bros. and OoT would drown in the selection. But I agree that they make it too slow, 3-5 games every week with at least a well-known good game every three weeks would be better.
Edit: On topic, SNES on 3DS eShop? I wouldn't say it's impossible now that there are already NES titles on 3DS, and there's coming GBA and DS titles for Wii U. I would love to be able to play Super Metroid on the go. But again they haven't said anything about it.
Yep.
