Cause the ASIC firmware doesnt have much capabilities, all the intelligence being hardcoded into wire and chip.
ASIC are not programmable. Once it is designed for a specific task, it can only do this task (very efficiently in term of power/energy). So if you change the task, you need to design a new ASIC.

FPGA are between too much general design like CPU which are made for doing a wide variety of task by implementing instruction logic and therefore full programmable, and ASIC which are very stupid chip which are only made for one task.
FPGA are often used to develop, test and design ASIC circuit. Which lower the developpment/test cost.
But market is a very small nich so FPGA are very expensive.
So FPGA are a kind of electronic circuit emulator, which of course will be less efficient than final asic implementation, but still more efficient in term of power/energy than CPU.