There's no proof that a $5M goal would make the campaign any stronger. Don't assume it could have raised $5M in one day just like it raised $2M. That's a ton of money and is extremely difficult to raise.
Imagine how you would feel if the goal was $5M from the start, there was no difference in funding momentum, and now you might end up with no game at all. Not only would the Kickstarter fail, but it would kill off any hope of seeing another game in the series because then it would be known as "that game that couldn't even get Kickstarted."