Usually if a product (like a game) turns out to have a lot of issues or maybe even being completely different from what was promised, less people will end up buying it.

However, preordering means people have already paid for the product before even finding out about any issues it might have.

Add in review embargos, and companies are basically "tricking" people to pay for something which may not be as good as the ads are trying to convince you of.

While you could argue for the fact that usually any issues that occurs in games are not big enough to really be a dealbreaker, the problem that people is worried about is that the more people preorder, the less publishers will focus on QA and making good games. Instead they'll move more and more towards just relying on hype and marketing to sell games before people find out that it's a really shitty game.