People play games ultimately as time killers. It's pretty much for the same reason that someone would go see a movie or pick up a book. Exactly what makes one time killer more appealing than another is ultimately up to the individual in question. Some people just want something they can be ridiculously excited for, and pretty much buy into anything with enough PR generating hype. Some people want new and innovative things. Despite what the internet might tell you, these people are in the minority. Most people are bound to pick up a sequel in a series they know they like rather than take a risk on a new IP that might not really be their cup of tea. Of course, if it's the same thing over and over again people are going to get bored. I'd say that most people want games that are familiar, but different enough to be interesting.
As for what sells games, I'd say that hype pushes most sales. Never underestimate the profit that can be generated by giving bored teenagers and twenty something year olds with disposable income something to talk about. Although I feel like with the social media phenomenon devs have to actually back up the hype with a quality game due to how quickly word spreads. So if your AAA million dollar blockbuster ends up being garbage people find out quickly. Not that it stops companies from doing hyping trash anyway.
