The simple reason is you need money and loads of it for research which they lacked.My dad is a metallurgist and worked in many countries including USSR. He taught at at Mc Master university Canada and as far as he is concerned the USSR was unmatched in propulsion and metallurgy.
After the collapse of Soviet Union and resulting hardships they fell behind significantly To the point where they relied more and more on mechanics and alloys ( cheap) but less on electronics (expensive) .
That was the start of their military decline. World moved on with smart technology and to this day you see Russians excel in maneuverability of their planes and suck in electronics .
So if PAF gets SU-35 then US should have no issues with giving us F-16's
For that very reason---friends of the Paf told Paf in the 80's not to go for the second batch of F16's---but go for the Mirage 2000---.
They told the Paf upfront---" you already know you will get sanctioned in due time---but if you get an aircraft that does not restrict your flying capabilities---the sanctions will be useless and they will not be imposed---".
And even if they are imposed---you will not be effected as much---.
Very true . One can see the world sanction has not been used since 911. I guess they realized how devastating it was for their relationship and realize with JF17 we are pretty much free of any fears of sanctions.
Credit goes to who ever started and took to completion our JF17 project. We can breath easy. Not to mention with Chinese technology maturing fast the day is not far when we won’t meed American latest technology either.