Perhaps SNK doesn't have the later releases available for download due to the sheer size of these games, especially towards the latter part of the system's life. Most of the games released from 1999-2004 were 708 megs, which includes such classics as SVC Chaos, Metal Slugs 3-5, etc.
Or maybe SNK is trying to play it safe by releasing the titles that the general population is more 'familiar' with instead of trying to satisfy it's hardcore fan base. The Neo Geo has always been a niche system, why not tailor to the wants of their REAL fans? I'm sorry but the first entries to Fatal Fury, Art of Fighting and Samurai Shodown respective series are a little too primitive compared to their much superior sequels. Windjammers would absolutely rule 2-Player online versus mode.
I also feel that SNK is cannabalizing their products by releasing these games online which are readilly available on compilations discs for much cheaper prices.