Speed. Reconfiguring an FPGA is a very slow process, and FPGA's can't match the clock speeds of conventional ASICs. That may change in the near future with FPGA-like devices built using memristors, which potentially could be reconfigurable at the speed of writing RAM and run at the same clock speeds as any ASIC.
That's marketing hype. If you reconfigure just one gate it can be extremely fast, but typical FPGAs take a significant fraction of a whole second to reconfigure. Though they are getting faster over time.
