I'm no expert, so I could just be saying pure BS here, but I have 3 ideas why it's no longer selling.
1. I only bought Pro Skater 3. It was great. It was fun. It was enough. Why would I buy any later versions ? 3 did the job well enough.
2. Isskateboarding itself as relevant as it was ? Back in the day yea it was definitely relevant. But now a days skateboarding just seems like just another thing. People still do it. But I don't remember hearing anyone talking about it when I was in school. Yea I knew a few people that did. But the most I ever heard was "wanna skateboard to the mall" or whatever.
3. Toney Hawk, I'm no follower of the skating scene. But I'm pretty sure he's not exactly a top figure anymore. Like John Madden, his name was put in the title when he was relevant. But now it's just a recurring punchline that the game can't get rid of because it woukd cause confusion.
All I can really say is that maybe skating video games just arnt popular anymore. Wait... That's it. Nobody WANTS to play these games anymore.
PATCH ME THROUGH TO THE KING OF VIDEOGAMES !!!