I think the main factors are:
-High production cost
-Proliferation of other marketing tools (video, social media, gaming press etc)
-Risk of alienating potential customers by letting them find something they don't like, rather than walking them through in a controlled fashion (videos).
-Community acceptance of beta versions which have the ready-made excuse of "it's only a beta!". I've lost count of the number of times down the years that I've seen people on forums coming out with these vain hopes that performance optimisations are suddenly going to make a beta that runs like a dog into a silky smooth affair. In my experience 90%+ of betas are indicative of retail performance.