Why was the Sukhoi Su-47 Berkut never adopted for service?
I do not have inside knowledge, but think it is easy to explain why the Su-47 did not progress beyond the prototype stage:


Sukhoi enjoyed great export success with the Su-27 and its derivatives in the 1990s, and this helped to advance work on the Su-47. Mikoyan was in less favorable financial shape during this time, which explains the delay in getting the 1.44 ready.
There was no credible threat that required Russia to procure a modern fighter design during the last two decades. The existing fleet was simply good enough.
The forward-swept configuration is optimised for close-range air combat, while the cost of modern combat aircraft dictates that they need to fulfil multiple roles. Note that the PAK-FA will replace both the MiG-29 and the Su-27 and their derivatives, so even Russia will not support two frontline fighters in the future.
Much has been learned in the meantime about stealth, so the Su-47 was already outdated by the turn of the century.


I have to admit that when I first saw the Su-47, I was scratching my head and wondered what the Sukhoi engineers were thinking when they proposed this design. But I had similar reactions when the Boeing X-32 was unveiled …



Edit: Now it seems the engineers at Sukhoi put what they learnt on the Berkut to good practice when designing the SR-10 two-seat trainer. The reason for the forward sweep is excellent handling at high angle of attack - however, this bleeds energy quickly and can only be used in few corner cases in air combat. But it certainly helps for pilot training if the possible angle of attack regime is large.