Almost all of the PS1 games available on PSN are Sony published titles, because other publishers seem to believe that $5.99 for one of their PS1 titles is asking too little.
On top of that, the PS1 emulator on the PS3/PSP appears to only support single-disc games. (Notice Wild Arms 2 and Syphon Filter 2, both prominent Sony published titles, have never appeared on PSN, yet Wild Arms 1 and SF 1 and 3 have). Many good RPGs from the PS1 days were multi-disc, including FF7,8, and 9, and Grandia.
Games like Tales of Destiny, Suikoden, Xenogears (which did show on Japan PSN) the Tomb Raider series, etc. appear to just not have publisher approval to go there. Suikoden has gone as far as to appear on the ESRB's list as games for "PS3/PSP" (i.e. PSN), but never actually made it there -- apparently Konami changed their minds?
If Sony ever does release a full software emulator for the PS2, on the PS3, you can bet that Sony published titles like Ratchet & Clank, Jak & Daxter, God of War, Gran Turismo, Mark of Kri, Wild Arms 3, Dark Cloud, Rogue Galaxy, SOCOM, Killzone, etc. etc. will show up, probably in the $12 to $15 range.
