Because it's slow? Not trying to argue, but show me a better zoom lens that can reach 300mm and give sharp photos for under $500. Sure you can get a faster lens, but you'll be in for around a grand or more. For the money, the 300mm is a steal in my opinion and it's speed is the only downfall. I've seen hundreds of incredibly clear photos with the lens, including birds and other wildlife.
By the way, I'm being serious. Please show me if there is a better lens under 500 that covers similar focus range, and AF-S or HSM. If there is a better lens for the money, I'd like to find it before making my purchase in a couple months.
It all depends on what you use it for. If you need a telephoto lens for indoor things, or for when there's low light, the 70-300VR is most likely not the lens for you simply because it is too slow.
I'm hoping to get a new telephoto lens because I need a faster, better lens than what I have (70-300 f/4-5.6G) for indoor shots because I just started doing some shooting for a newspaper. I'm thinking about the Sigma 70-200 f/2.8 simply because its cheaper and a little bit smaller than the Nikkor version. (I don't have tons of money to spend on nice glass.)
I've also used the 70-300 outside before when it was far too slow to capture any birds or anything. Even without the extra 100mm, a 70-200 f/2.8 is often a better choice simply because it is faster and has better IQ. If you want to freeze motion (often what you need to do in bird photography) a faster lens is essential. Granted extra focal length helps, but if your initial image is sharp and you have good IQ, you can crop a little bit. In bird photography, 300mm a lot of times isn't enough. Many big birders use 400mm+ primes and rarely use zoom lenses.
Granted my personal experience isn't with the new 70-300VR which has better optics and AF-S which certainly both make it better than my version, but it is still a slow lens (even slower than mine by a little bit) for a very large focal range/length.
Also, just because a lens is cheaper now doesn't necessarily mean you'll be saving money in the long run. If you think that you will be held back by it at some point, spend a little more and get something better.
If you don't think you'll ever be held back by the 70-300VR then go ahead and get it--don't let my posts stop you. I'm simply saying that from my research and personal experience a 70-300VR isn't always worth the money (like the 18-200VR), and that's why I ended up not buying one when I almost did a few months ago (both lenses actually). It all depends on what you want to pay, what you expect from your images, and what you expect for your photography in general.