Regarding Percy Jackson 2, yea, they took a long time to make a decision to greenlight a second one, so, it seems TV Rights and/or DVD Sales edged it into being Greenlighted
I also agree, that HP, Twilight, and Hunger Games had the wider Mass Age Group appeal (And as mentioned, HP Movies and Books running together, likely helped fuel it so well). I also think having JKR on hand for approvals also helped ensure Readers were satisfied for the most part with the Adaptations. If you have a Huge Reader Base, and you piss off the bulk of them with your Adaptation, that's a lot of bad press and bad word of mouth to overcome.
The Fans would likely be Gung-Ho when it was announced, and telling friends "Well, now you can watch it, instead of having to read it" and "Go with me, you always seem interested when I talk about the Books" and so forth, but, as Spoilers are leaked, if something the bulk of the audience got cut and it pisses them off, they'll spread that dissatisfaction. If your readership isn't Huge, that makes it less risky in this area, but, also less fan base word of mouth if you do it right by them. You're probably always going to piss off a certain at least small percentage, that almost can't be helped