This is important for eSports. Having to learn a new game every 3-4 years so you can follow along is difficult for viewers. It also allows professional players who invest the training to transfer their skills more easily.
It's different with other companies -- Blizzard, for example, created a new game in StarCraft II. It shares a few similarities with Brood War, but the differences are vast. I really, really like Valve's approach.