Because it's easy to say something is shit without ever seeing it. When you hope a movie is going to be great, you're investing a little bit of your optimism and effort into it. When you say it's going to suck, you just don't care, so why should we care? Some people say they care so much they think it will still suck, well apparently such persons don't care enough to see it before saying it will be awful.
We've just got better at cynicism as information comes out:
New ghostbusters woohoo!
It's a reboot...hmm..That's worrying (if you don't like reboots or recent reboots) but hey New Ghostbusters woohoo!
It's a deliberately genderflipped reboot and noise is being made about this! (loses some people, gains other, plenty of publicity, but now politics is partly in control of a creative process, which may lead to worry...)
Trailers come out, there's outcry, some things are great other things not so much.....(now the positivity is really wobbling, because there's even more variables in the mix....depending on how much you find to like/dislike)
Information, and especially trailers, on hoped for films can be a double edged sword. (prometheus....predators....star wars...jurassic world)
They either reinforce positivity or enhance the doubts.
Ultimately, yes, the film is still gonna go either way and be the ultimate decider, but with each bit of information released, the initial excitement is going to be tempered by added uncertainty and thats going to vary person by person especially when making a franchise film with an existing fanbase.