Why are SRAM based FPGA used more than NVM based FPGA?
The main driver is the fact that SRAM is highly compatible with the same physical process that is used to implement the actual logic. Indeed, most FPGAs these days are based on LUTs (lookup tables), which are really just tiny bits of RAM themselves.

On the other hand, the process required to build EEPROM (nonvolatile memory) requires extra steps &mdash; to create floating gates with special oxide thickness, etc. This process is NOT directly compatible with the logic/SRAM process. This means that nonvolatile FPGAs are somewhat of a compromise in both areas.