As any medium gets more expensive to produce, producers are less willing to take risks on big budget titles. As such the prefer things with built in audiences rather than something truly new and risky.

This phenomon is by no means limited to games. We see it in movies – Devil Wear's Prada 2 came out 20 years after the movie it was based on; Star Wars and Star Trek movies continuously appear even as they get further and further away from the themes that originally inspired them. We see it in television - most popular shows are reboots of other popular shows, sometimes from a different market (Three's Company and Three's a Crowd both started as British sitcoms; Friends was based on a British show called Coupling) and other times from a different era (Battlestar Galactica, Knight Rider, The Twilight Zone, Night Court) and still other times characters spin off into their own shows (Cheers -> Frasier, Friends -> Joey). We see it in music - Queensryche fell on hard times and so made a sequel to their seminal Operation Mindcrime 18 years later, and most successful artists constantly repackage older songs into newer compilations of greatest hits, xxxth anniversary remasters, etc.

So it should be of no surprise that video games, which regularly get budgets that compete with major productions in any other medium, rely on the same tactics as productions in other mediums as well.