Those games are played by a demographic that only plays that game, or close enough. They'd consider themselves a Dota player before they consider themselves a video game player in general. These games aren't played exclusively by that type of person, but a large part of their audience is the type of player who just plays that game. I'm having trouble digging it up, but the person who created Steamspy a number of years ago, before privacy laws made public profiles opt-in and interfered with its ability to collect data, found that the majority of Steam accounts only had a single game in their libraries.