Because people tend to listens to what other people has to say.
Ironically, people do indeed take all the negativity for granted, but they completely ignore all the positivity that reaches them in the exact same way. I fail to understand why exactly this keeps happening.
If you are subject to this form of manipulation, it should work both ways, but it doesn't. The only thing that comes to my mind is manipulation. The average person always waits for a movement in order to act.
If whales initiate a dump after bad news, people think it's affecting the market because the price is going down, so they sell. If whales decide not to pump after bullish news, it means that people won't be buying.
It's makes sense, but there might also be a few other factors that play a role of importance.