Designing an ASIC with low power in mind will always beat an FPGA, no question there. But the design cost is prohibitive for many applications (not to mention the lack of flexibility to re-iterate/patch your design with little to no cost). But compared to GPUs I'm fairly certain you can do much better power-wise going with an FPGA. You have finer control over your frequency, over which sections to power down, how often to switch, etc.

Of course you can get better price/GFLOP with GPUs + quicker time to market

And yet they (GPUs) aren't (better for FLOP/watt than FPGA) everywhere.

GPUs have very particular cache and computation hierarchy which is not necessarily a best/good fit for all problems that are being thrown at them.