Adding programability to hardware always adds complexity and thus overhead. FPGAs have significant overhead when compared to ASICs. The difference is in the amount of components inside (transistors, wires, building blocks), distance of the components (how much time does it take for the signal to go through), maximal frequency (100-400 MHz for FPGAs, 1+GHz for ASICs), power consumption. For mining, the overhead is the key here.

With FPGA technology, you must adapt your circuit to the properties of certain chip family, its size, its building blocks, its maximal frequency. This is different from ASIC where you can in principle choose those properties, i.e. you adapt the technology to the needs of your circuit.

I believe that using photons instead of electrons just makes the signal transmissions faster. I.e. both FPGAs and ASICs might get faster. But the overhead does not disappear. To have a good reason for using programmable chips instead of hard-wired ones, there must be a significant advantage that over-weights the loss from higher power consumption or lower frequency or lower performance. Something like significantly cheaper FPGA chips that can for the same price over-weight the equivalent ASIC. Theoreticaly, you can catch it on the performance axis. But the power consumption would be a big issue here.

I am not expert in the photonic technology.