It's the hype.
95% of the people believed it and now they're pissed.
The BSI appears more of a gimmick as the noise level is the same as my 5Dsr even though it has more MP and 'ancient' nm process. The Canon AF compatibility that was so heavily touted ignored the longer lenses and is still slow as fudge. The thing overheats in 4k unless you have 27mph NW winds, one foot off the ground and a bucket of ice handy. Silent shutter mode creates banding at completely random times. Plus it's massively over priced.
That's bad enough to warrant negativity I guess. And this can become infectious.
Then comparing it to other brands with better, tougher intuitive interfaces and menus, decent flash systems, native lenses and people feel hoodwinked en mass.
You could say it's the consumers fault for being consumers. I've never seen so much hype over a camera and I'm totally over getting one now. (Although I am eyeing a 7s for the ISO performance - but even then there's no point shooting high iso if the images are all washed out anyway).
I do think though that what Sony HAS done is to show that certain levels or technology are now available and this drives other manufacturers forward.