Because that's what the contract is for the top three series. NASCAR uses Hoosier in the Modified division and General in K&N, ARCA, Pinty's, Mexico and the Euro divisions.

Regarding cost: would you really trust a tire made cheap enough to cause significant savings over the course of a season? Even if a rash of safety improvements would negate the injuries of the last round of tire wars, you still would wind up with a bunch of wrecked racecars. I would assume any competing manufacturer that made tires for Cup cars at a price point cheap enough to cut costs significantly would wind up with a 2008 Brickyard scenario before I would assume they'd overtake Goodyear.