One of the pro reviews said that the reason they did not put in a campaign was that they simply ran out of budget, or it would not fit in the budget, something like that. However that may be, it is true that all development time going into a campaign would be development time not going into the game as we have it. A good campaign takes a lot of money to produce. A mediocre campaign is not worth having.
The real shortcoming that I see is not a lack of a campaign, but rather Tutorials were too brief and superficial. Not all new players are familiar with either 4x games or space combat games. I am familiar with both; just saying that many who purchased the game were probably not.
Thinking back, I tend to glance over the manual if there is one. Do Tutorials if there are any. Then the first few missions of a campaign. Usually after the first few missions I hit the skirmish against AI mode and probably never finish the campaign for most games.
As for what the Vasari are running from, not an explosion of the galactic core because they are running at hyperdrive speed and a core explosion wave front would only travel at light speed. Maybe Sarah Kerrigan is chasing them!