It's because a Macbook is a Macbook whether its 2005 of 2011, but some HP-24x454 vs a MX311 is meaningless. That's why Motorola/Samsung are making their Droid/Galaxy lines.
In the PC world your pricing competes against every manufacturer. So HP has to push their prices down on next rev.
Whereas no one else can make an Apple computer. They can set a price and keep it fixed for a year. There's no knockoffs that can undercut them.