It's a common, but in my opinion misguided, marketing plan to put out a low quality product to remind people your franchise exists.
For movies, its to maintain the license. For example, unless Sony releases a new Fantastic Four, X-Men or Spiderman product every X years, they will lose the rights to it. This situation obviously doesn't exist for Blizzard and Diablo.
And how much of a brand is Diablo, anyways? Its just one product, just a video game. Not like they have movies, animes, comics, novels, toys, and so on, lined up for release. And not like video game fans forget about a property just because its sitting stale, Baldur's Gate 3 releases how many years after Baldur's Gate 2? And at a period of no other successful Forgotten Realms titles. If Half Life 3 releases tomorrow, despite lack of bad games keeping the property fresh, people will flock to it.