At the moment the OS is very tied to specific hardware. This reduces the complexitity of development and keeps the design team lean mean and innovative. Once they see major marketshare gains however they'd have room to compete in this way.
Releasing it now would cost them valuable development hours and convince Microsoft to go into an all out drive against them (and MS is never afraid to LOSE money to kill off a competitor.
For now its probably best to let sleeping 'dog of an os's lie.
Apple is a hardware company. Their OS and other software enables them to add value to their hardware, distinguishing it from the other PC's on the market.
Microsoft is a software company. Their business model ties their software offerings and the OS in a way that enables them to sell more of both.
Two very different companies.
Also, if Apple were to recoup the expense of its software R&D, if they sold a Mac OS X version for generic PCs, they'd have to raise the price of the OS. They'd also have to spend more time developing the OS for other hardware platforms. To be sure, they'd probably do a good job, but considering the user costs of switching, it is unlikely that they'll lots of attract new users by allowing users to install the OS on a generic PC. Far more likely, the end result will be a decrease in Apple hardware sales, thus killing the company's primary revenue generator.