Electronic – Why are SRAM based FPGA used more than NVM based FPGA

Presumably the FPGA has some purpose for its SRAM connection, though you haven't specified what that is. I would guess that your best bet would be to have a means by which the Cortex can sufficiently influence how the FPGA writes to SRAM, and find out enough about what it's reading, to determine whether the SRAM is working properly. For example, if the FPGA would normally write the SRAM with data captured received from an ADC, and output it, after a delay, to a DAC, one could have a test mode that would instead write data from a linear-feedback-shift register, and another test mode that would mix data read from the SRAM into a linear-feedback-shift register with slightly different taps. If one can arrange to have each test mode run through the SRAM a controlled number of times, initializing the LFSR before the first pass and reading it after each pass, one can arrange things so that it would be unlikely a broken SRAM would yield the correct LFSR values, but the hardware could be simpler than allowing the CPU direct access, and (more importantly) the access timings would be like those the SRAM would see in actual use.

No fundamental reason why not. Synchronous SRAM is truly random access, fairly inexpensive, and easy to interface to.

Its downside in that it occupies a fairly narrow niche between the on-chip BlockRam (not much smaller, free until it forces you to select a larger chip, massively parallel and more flexible) and external DRAM (massive storage capacity at a price SSRAM can't match).

So up to 0.5 or 1MB, external SSRAM is unnecessary, and above 8MB or 16MB (numbers may vary according to your budget and current prices!), SSRAM becomes expensive enough that DRAM takes over despite its limits. Then - if you need random access - you have to massively reorganise the computation to read chunks (bursts or pages) from DRAM into BlockRam where you can process it fast before writing back bursts etc....

But if you have a role for SSRAM within that window, go for it. I have added simple home-made SSRAM boards to augment commercial FPGA platforms where necessary.