I think partly it's that, for whatever reason, some people just completely dislike the idea of multiplayer-only games with no singleplayer component, despite the fact that no-one bats an eye at singleplayer games with no multiplayer. And I can kinda understand that, if you don't consider yourself to be "good" at multiplayer games or just like playing stuff at your own pace rather than have so much of it dictated by other people or whatever, then multiplayer games might not be very appealing to you.
Add to that the fact that as a result, this game seems to be very "simple" and arcadey (in a good way, imo) and with the story aspects serving more as a backdrop to the action rather than the focus, like they otherwise might have been in a singleplayer game, and I think some people might just be looking at it and thinking there's not much there.
I get that, though personally I'm very much looking forward to it. I loved Left 4 Dead, very much enjoyed the recent Back 4 Blood beta, and think this looks like a great PvE multiplayer game in the same mould. Not to mention it looks extremely polished, and all the reactions from people who've played it have been positive. So yeah I'm definitely hyped. It's been too long since we had a good game in the Aliens vein, so I can't wait to get my hands on it.