Posted this in another thread, but figured it was worth reiterating here.
Note that I have never released a PS2 Classic on PS4, but here is how I understand the process works:
1. Sony does the port, testing and QA
2. Proceeds from the game's sales all go to Sony until it recoups Sony's costs
3. After that it follows the standard 70/30 split all PSN sales do
I would argue that it would be impossible to make it any easier than this for the publishers involved, since it actually costs them nothing up front and has zero risk.
The system was designed to get as many out as possible, but publishers aren't taking them up on it.
I suspect it's because most of the PS2 games people care about are Japanese, and Japanese publishers would rather try for a rerelease themselves, or want to charge an unrealistically high price.
This is pretty informative if that's how it works.
But yeah, I had always just assumed most Japanese publishers have no interest in it as a lot of them (Square Enix in particular) prefer doing remakes and re-releases to begin with.