Because the way a console launches and the way it is recieved early on can have a big impact on its future sales. When you generate a ton of hype around launch (like with the wii or an apple product) it gets every talking about it and gets more people interested. It is far, far harder to achieve this outside of the launch period. This is why peripherals released halfway through a generation tend to struggle.
Also i think the bigger issue is that we are still waiting for the big games to come out. If you don't have the big games to push the system and you can't advertise your product because your worried people will be disappointed and sell it, why are you eve releasing it yet?
I think it is clear Sony was too confident with the product and over estimated the market interest and ability to purchase and invest in a next-generation handheld, especially one with mainly niche strict followings.
However, the fact that they realised the market isn't interested at the moment gives them all the more reason to hold off a huge advertising push showcasing what is mostly the same launch games. When they are confident in their upcoming lineup I'm certain they will bump up the ads. In fact, I'm hoping from a little before the new big games come out till the end of the year they flood vita ads. Coupled with a price drop, especially of memory cards and the flock of games coming out which will give them great advertising footage, it's hard to be too worried. Like I said, they need more of a push then the current lineup at the current price.