The original release date was in 2014, however. It's hard to fault people from getting a bit disillusioned now that we're mid 2017 and completion is nowhere in sight. Yeah, progress is being made and all, but the pace is pretty slow all things considered. And I assume many people joined post 2014, but it doesn't change the fact that it had a long and, if a quarter of the shit being throw around the 'net is right, pretty rocky development, and even alpha releases keep getting delayed too.
No Man's Sky also probably made some people more skeptical. Yeah, the context isn't the same, but the basic idea of a (potentially) overpromising and under delivering space sim remains the same.