That is because everyone knows that ASICs are the inevitable end game. Whoever has those wins.
Even a ASICs design will have a hard time vs a few million gamers worldwide who consider their graphics cards free and GPU shrinks that happen every 18-24 months on average. Any ASIC design brought out today will have to compete with the radeon 7xxx series and not current hardware.
The problem is that an ASIC can be custom designed to go orders of magnitude faster than a GPU for a specific task. As just a very rough estimate, one SHA256 engine with bitcoin hash difficulty detection would take something like 1 million transistors. The latest ASIC technology allows 2 billion transistors. So that would allow 2,000 of these engines per chip. If you layed them out by hand, you could probably get them up to 4 GHz. Assuming one hash per clock, this ends up being 8 TH/S per chip. In other words, this one chip could outperform the entire current BitCoin network. And once the multi-million up front tooling costs are covered, you can make each chip for $50 each. So for another $10,000 you could make a system with 200 chips that would be able to dominate the entire network at 200 times its current capacity. It would pay for itself in about 20 days, because you would get every bitcoin made in that period.
If the exchange rates hold, you could multiply your earnings and buy thousands more chips for a low incremental cost to maintain your dominance. I don't see how GPUs could begin to compete with this.