Actually, the big break between SMIC and state-of-the-art was very recent. Up to 2015 SMIC was catching up extremely fast.
For example SMIC was on 28 nm in January 2014, having completed 28 nm R&D in 2013. TSMC was
also on 28 nm in January 2014 and was planning on transitioning to 20 nm at the end of the year.
So at that point SMIC was just
months behind the state of the art.
Qualcomm (!!) was fabbing with SMIC as a leading edge fab in 2014.
However the big problem was finFET which TSMC got first but took SMIC
4 years to figure out. Then the really big break was with EUV enabled nodes which SMIC was denied due to political tricks.
So it wasn't due to incompetence at SMIC - they were closing the gap from 2 years to mere months with the leading edge, but then got hit with a paradigm technology shift AND a tool shift at the same time.