Ehhh not really. There was some FPGA mining that happened in Bitcoin but it was always niche rather than at-scale.
FPGAs are a lot better than GPUs from the perspective of energy efficiency, but the FPGAs themselves are very expensive to purchase, and are a lot more specialized than GPUs. ASICs showed up before FGPAs ever really took off, and even if ASICs hadn't showed up it's not clear that FGPAs would have been superior on a total-cost basis to GPUs except in niche circumstances.
What do you mean by "more specialized" ? FPGA's can implement any logic circuit whereas GPU's are optimized for highly parallel arithmetic computations. I don't understand how that makes FPGA's "more specialized". It seems like they are in fact more general.
They were, before the ASICs came. CPU -> GPU -> FPGA -> ASIC. A classic story of specialization.