Hello,
I think that many AAA games are sold by hype. They cannot convince through good gameplay or narration so they must use hype to sell their buggy, unfinished, uninspired, hollywood-like cutscene fest product to the unaware sheeplike consumer. So gaming journalism helps to build up a level of hype around the new game. That's why many resources are spend in marketing. Resources that would be better spend in developing good games. Todays target audience does not buy products for the value or quality, they buy the reputation a product comes with it. When it is cool to own the product and everyone they know owns it, they buy. To establish this kind of name regocnition, you need a ton of marketing, advertisement, game journalism bribes, game convention in luxus hotels, porsche test drive invitations for journalists, product placement in gaming shows, super bowl halftime commercial spots.
A demo version of the game can destroy the overexpectation, that a pontential customer has, crushing the hype under cruesome boots of reality. Oh the new Railwayshooter sequel x+ has the same gameplay as every other shooter out there. So demos are contraproductive to sales when you try to sell a mediocre products.
Back then when games had to convince aware consumer through originality and good gameplay, when developers were convinced that their products weres good and proud of them, demos were attracting customers. Nowadays demos are chasing customers away from mediocre products.
Demos cost money and manpower, those resources are already spend on cutting existing content aka developing DLC for the game.
Big AAA publishers simply have no incentive to make demos any more. Demos are not part of the money making plan.
Some developers like Gearbox produce fake demos and trailers to mislead their potential customers. Like they did in Aliens Colonial Marines. #
http://www.g4tv.com/thefeed/blog/post/727861/aliens-colonial-marines-gameplay-video/#
Have a nice day.