On July 17th, Tony Hawk's Pro Skater HD will be leaving the Steam store forever. Until that time, the PC port will be available on Steam for 80% off, giving it a $1.99 price tag. Though no reasons were cited for the game's removal, the most likely reason is music licensing. The Tony Hawk games are known for their soundtracks featuring music of their respective eras, and the first game introduced a lot of fans not only to the sport of skateboarding, but to bands like Dead Kennedys and Primus.
Unfortunately, the original soundtrack was gutted for the Steam release due to, you guessed it, licensing. That, coupled with a myriad of other problems, caused the HD makeover to feel less than the sum of its parts, and far from the quintessential experience. Another possible cause is because of Activision, a company that holds the video game rights to many established characters, and a history of letting those rights lapse, resulting in the game being pulled off shelves, like with Deadpool.