The series has had 10 releases in 10 years. There's really little point in rereleasing the 4 (or 6 including the PSP ports) PS2 games in HD.
I'm quite sure all ps2 games will eventually be sold on ps4 though.
Then there was also little point in redoing other PS2 classics, but Sony still did it. I have the Sly series, Team Icos games, and Metal Gear Solid 2 & 3 on the PS2. Still, I want to replay them, and I plan on buying the HD Collections. When I get back from BCT for the army, I plan on spending a large portion of my pay clearing through my Amazon games wishlist.
I posted this on Sony's forums last night to show that I'm prepared to pay for a copy of the HD remakes. Sorry about the greatest hits cases on the 3 later PS2 games, I was in highschool and rather poor when those came out.