Because HP says the fact people are increasingly ditching PC's for tablets (read: iPads) factored in their decision :
"The tablet effect is real, and sales of the TouchPad are not meeting our expectations," Apotheker says, explaining the movement of consumers from PCs to tablets as one of the problems with the PC division. So H-P is exploring options for its unit that "may include separation through spinoff or other transactions."
Basically they're saying iPads are where the money and future growth are and they failed to create even a beachhead in that market so they're getting out.
That seems like a very shortsighted move because HP only acquired Palm last year and has only had a competitor to the iPad on the market for a month and a half. To kill your largest division because your 1st generation tablet didn't make an immediate splash against a product from a company that has a cult-like following tells me that HP's management isn't willing to work to develop a product that they purchased just to enter this market, and they deserve to fail as a result.
Also, Dell and Lenovo may take HP's share of the corporate workstation market and Acer the low-to-mid range laptop but Apple will probably take a healthy bite out of the more profitable high end laptop share (what HP had left of it) and high end PC. The volume isn't really important, the fact that there isn't any money left in being top dog volume-wise is the whole reason they are getting out.
While there isn't a lot of profits left to squeeze out of that market, it is still the company's largest division, and it's not like they have a lot of options to make up that income. When IBM sold their PC division to Lenovo, they had the market for big iron locked up, a successful line of midrange computers, a line of servers, several lines of enterprise software products, and a large consulting division. HP has their printer division, their server and storage lines, their line of Itanium products, and their networking division. HP doesn't have enough in those other areas to make up for spinning out their PC division.
The only thing that would make this move make sense is if HP is attempting to focus solely on the data center and gambling that VDI will be the future of the corporate desktop.