People have limited time and limited money and, while videogames are not the most expensive or time-consuming of pursuits, with how much of an investment there is in a videogame people are likely going to be careful with what they buy. What this means is that most people are not going to buy a videogame they know nothing about. Now, whether people are researching games online and reading reviews or getting their information from friends and family a game that is simply not fun will have difficulty getting decent sales.
On top of this, when you have a quality game you need to draw attention to it so that people bother to pay attention to it. The most common ways of doing this is to attach the game to a popular series and to advertise the game heavily. Even if you have a high quality game if no one knows it exists it is unlikely that it will sell well.
