After like, 8 years, there are only about 100 PS2 titles on PSN (and those are largely determined by what companies wish to participate/what resources they still have kicking around to port the titles). Why are people getting so uppity about something that is only going to get marginal support?
Believe me, I'm as mad as the next guy that I don't have easy access to the PS2's amazing library. A few years back I went to Japan to see how many awesome Japan-only titles I could pick up, only to find that many of them had low print runs (the Japanese market is super small, remember), and cult classics go for upwards of $200* since they never received budget reprints (like 'PS2 Best Collection' or whatever). I would LOVE to be get my hands on those gems for a paltry $15, but the fact is, most of those companies went out of business because they couldn't keep up with increasing costs of software development (the PS3 all but murdered Japan's unique small dev scene). The ones that survived either did so because they jumped to mobile or PSP, but in either case they dumped all their resources and no longer have the ability to do a proper port. On the anniversary of one of my all-time favorite games, people bombarded the company's twitter account with questions about possible PSN releases of their classics... only to get a flat out, "Nope, it's not possible. How about Kengo? You guys like Kengo, right!?" To say nothing of any rights and licensing issues...
Long story short, I'm glad they recognize the PS2's legacy, but the greater part of that legacy is lost to time. I know a lot of people are screaming "PCSX2!", but believe me, that experience still a shade off of the original console.
*I WAS able to find a handful of games I wouldn't trade for the world, though that was really luck-of-the-draw. They cost me $15-25 CIB, which seems appropriate.