Because the incumbent handheld market leader has announced a new device with massive 1st and 3rd party support. If the PSP2 is going to be late to the party it is going to have to bring something that will flip handheld gaming upside down. This time around, the Playstation name doesn't have that weight of presumed success or the position of being a handheld for older gamers. Those advantages are back on Nintendo's side.
Any advantages not held by Nintendo are held by Apple (extremely cheap and accessible games, a diverse range of hardware formats, excellent internet and multimedia functionality, and the biggest digital game distribution system anywhere).
From where I'm standing, the only advantage PSP still has is a solid market presence in Japan, but there's no reason to expect that to endure when the competition is moving forward and Sony is standing around.
"The worst part about these reviews is they are [subjective]--and their scores often depend on how drunk you got the media at a Street Fighter event." — Mona Hamilton, Capcom Senior VP of Marketing
*Image indefinitely borrowed from BrainBoxLtd without his consent.