It seems patently unfair for a prosecutor to announce a criminal complaint and arrest an elected official with great fanfare and then say a full 30 days later that he needs more time to build his case. The rule must be that a prosecutor should only bring a criminal complaint unless he already has a strong enough case that he believes should prevail at trial.Trying to read the secretive Fitzgerald is tough. Until the Lewis "Scooter" Libby case played itself out, no one could have imagined that Fitzgerald knew who the leaker was (and that it was not Libby) from the start, and that the whole investigation amounted to one big perjury trap. So I don't think Fitzgerald would be afraid of drawing Obama into the fold, but we'll probably never know.
As to the lack of evidence, Fitzgerald may have felt that he had enough evidence to charge on the crimes alleged in the criminal complaint, but has learned of new crimes. It is not hard to believe Fitzgerald's representation that he has obtained new information and witnesses.
In all likelihood, when we see an indictment, it will be even more wide-ranging than the criminal complaint, and will seek to have the Governor's office declared a criminal enterprise (I'm going to write more about this when I get a chance). Running a criminal enterprise (City Hall) was the basis upon which Providence, RI, mayor Buddy Cianci was convicted even though he was found not guilty of the specific bribery and extortion charges.