Because people were complaining that Wings of Liberty’s campaign was too long. Wait a minute, too long? What?
In an interview with Kotaku, senior designer Jonny Ebbert has stated that the reason why StarCraft II: Heart of the Swarm‘s missions were reduced from 29 to 20 was because they received multiple complaints from players and even from inside the company about the game’s length:
We actually got a lot of complaints that it (Wings of Liberty) was too long. Like a lot. We looked at the achievement data, and the completion rates just went [down]. We got a lot of heat internally about it as well. People on other teams were just ‘It’s too long!
I’m not exactly sure where they were going with this considering that some of the missions in the Wings of Liberty campaign were optional.
Sounds more like a business decision to me.