Google and LG are limiting the supply on purpose.
By releasing a phone that beats all other android pones in the value-for-$ segment by far, Google is competing with its own partners. It isn't wise to bite the hand that feeds you, so Google creates the hype without selling too many phones. Google isn't interested in making $ from Nexus 4, it only wants to create the hype, and show its partners that they need to do better (better value phones with up-to-date OS - which is what nexus 4 is). Google is basically telling its partners: "If you don't do a better job, remember that I have the potential to create a far better device than any one of you has right now, and you don't want me to do that and eat into your profits, now do you?" - hence the limited stock, because Google just want to bluff that it can eat its own partners for breakfast. Google wants as many partners as possible, and wants them to complete with each other, while google sits back and counts the $.