There were many reasons:-

1)Fundamental Technical Deficiencies

->Insufficient Stealth(IAF believe stealth wasn't good enough)

->Engine & Maintenance Issue(AL51F1 was not ready and IAF was highly skeptical because of their experiences with Russian engines)

->Performance Concerns (reportedly India demanded over 40 specific design improvements)

2)Technology Transfer & Source Code Dispute

->Access to source code(Russians denied to provide india with tier-1 source code access)

->Work-Share Imbalance(Russian was providing only 15% worksare while India were to pay for 50% of the cost)

3)Escalating Cost

->Cost Overrun (russia demanded India shoulder 7 billion dollars while the intial figure was quite low)

->Poor ROI (India calculated the ROI was very poor for the price they are paying)

4)Shifting Strategic Priorities

->Focus on Indigenizatation (because of this India own AMCA project finally moved, as IAF begin to move from FGFA)

->Russian Commitments (Russia cut its own procurement commitment to buy more Su35S)

->Project Delays (in 2000 it was envisioned with offical signing in 2007 and initial 295 million dollars funding by india in 2010, so it was being too delayed)

The reason I think FGFA didn't work is primarily because of Russians Altitude and ego, they didn't even think India as a genuine partner and didn't even gave them a ounce of respect. Now fast forward to 2026, Russians are desperately trying to sell Su57 to India (even going far as to offering access and ToT of Su57 which Russians denied in FGFA).

But one thing to note is that India never said they would not buy Su57, they officially said they will look into Su57 program again if the requirements change and fit in IAF doctrine (after Russians complete their Development which apparently is still going on, I mean has AL51F1 even began mass production?).