You forget the immense profits they gain from adverts and free publicity at eSports tournaments, etc. This stuff is broadcast on TV-channels and streaming-websites worldwide, and even sells out major arenas from time to time. The longer each iteration of Starcraft 2 is relevant and popular, the longer they can wait with releasing expansions, and the longer the game will be kept alive. This also pushes back the eventual development of Starcraft 3, which also saves them a lot of money.
In short, developers want their games to last as long as possible. That's the main reason they delay HotS and LotV. Implementing ten new units, a new campaign, and re-balancing parts of the game hardly takes an experienced crew over two years.