I assume that SegWit is so perfect,
That's a bad assumption to make. No software is perfect, no solution to scaling will be perfect.
but why most miners refuse to use it? It is 3 months after SegWit released, but only 20% miners agree to switch in SegWit mode, what is wrong? What do they think?
A lot of the problems seem to stem from miscommunication and lack of communication between developers and miners. It seems that some miners have taken a grudge against the developers (all of them in general) for not consulting them when devising scaling solutions.