It's sort of a mix between all the topic answers so far.
One of the biggest factors (over the past few months anyway) has been both fear and speculation...coming from the whole notion that "if you don't preorder your device now, the difficulty will be too high later and you won't make any money".
During the first preorders for both BFL and Avalon, purchasing an ASIC was more of the "logical next step" for those people willing to continue mining on a large scale. It was only matter of time that custom chips would be designed specifically for the purposes of processing SHA hashes for Bitcoin. It was time for a mass move to faster and vastly more efficient hardware, compared to CPUs/GPUs/FPGAs. The massive increase in speed that thousands of distributed ASICs would bring to the network ultimately help to secure it.. stabilizing Bitcoin a bit.
When more and more people started to preorder and prepare for the transition, we (as a community) began to wonder what the future of the network will look like. How fast will it be? How high will the difficulty be? How many people are purchasing ASIC mining gear? Etc. Asking these questions, and subsequently attempting to seek answers, helped to drive fear into many people who were not necessarily looking to become early adopters...resulting in more ASIC orders.
And then there's the potential profits for those who buy in early. If you're able to get your device earlier than the majority of others, you stand to make quite a bit of coin..more than enough to pay off your initial purchase in a timely manner. One of the first ASIC owners, for example, has been pulling in nearly 60 Bitcoins per week for a month now...vastly more than enough to pay for the $1300 Avalon rig they purchased. Imagine what you could be making at 60GH/s on a 30m difficulty if you bought in now, compared to a 50m difficulty if you buy 2 months from now...that feeling of "
oh, I should buy now" is what drives people to "rush" for ASICs. Timing is everything
TL;DRWhy does everyone rush to buy an ASIC?Overarching theme:
Fear / Speculation and the "Get in Now" scenario1) ASICs are the logical next step for miners.
2) Difficulty growth and total order speculation
3) Potential Profits for early adoption