The problem with the single player campaign is pretty simple:
People don't want 30 missions for a single race, they want a little bit of all races. Instead of molding the story to fit the customers' desires (which Blizzard has always done until recently), they built the game around the story, ending up with dissatisfied customers.
Can you imagine Starcraft 1 not having a Protoss or Zerg campaign at all? That's essentially the problem here. You cannot simply assume people will buy the next game for the other campaigns, as that is admitting to Wings of Liberty being an incomplete game.
Unless they're building the lore up for an MMO or something, I can't see the reasoning behind this. I personally don't care as much, since I'm in this for the multiplayer, but I believe Blizzard themselves have stated a significant amount of people buy their games solely for the campaigns.
