Lack of money. Russia's defense budget is only a fraction of what it was during Soviet era. Developing a brand new jet fighter costs tens of billions of dollars and decades of research. Russia's economy also isn't going to get better anytime soon with the sanctions and falling oil price.Why did they not design an S-duct into this fighter. Does it mean the fan blocker is definitely effective? Western commentary on this S-duct issue seems to suggest that Russian engines are not able to deal with the "turbulence" from S-ducts or they just simply can't do it. This is obviously hard to believe given how many other fighters in Soviet and Russian arsenal had S-ducts, not to mention the J-10 and J-20 both fly with S-ducts while using AL-31s.
Seems like a real oversight but there's gotta be more to it. Radar blockers? Offset fan surfaces? If it's possible to design stealthy bomber and fighter airfoils, it's possible to design stealthy fan surfaces. Personally I think given the slightly offset angle of the engines and the position they occupy from almost all frontal angles, the blades are designed to bounce energy off some internal sections which either absorbs and/or reflects the energy further away from source.
To put into perspective, Russia has not constructed a single warship from a destroyer class and above. Most of Russia's hardware such as T-72, T-90, Su-27/30/33/35 series, Mig29/35 series, were all developed during Soviet times. After Soviet Union dissolved, Russia's economy was crippled, however it inherited vast numbers of weapons and defense research whose costs were written of as a result of the collapse, benefitting Russia immensely.