Usually DA's are very loathe to bring charges unless they are very sure they have at least a good chance of winning, (they run for office on their conviction success rate) so I would imagine that the prosecution is holding back evidence from the public that can be used to their advantage, although sometimes politics, arrogance and over-optimism can cloud their judgement. It seems Cosby's accuser is also suing the former DA for dismissing her original claim and not prosecuting, so this gives Cosby's defense even more fuel to try to muddy the waters, which is exactly what defense attorney's are paid to do, create reasonable doubt.
As the other civil cases are not yet litigated, they will take a back seat and they will have no legal bearing on the criminal case, although the civil cases will undoubtedly be in the minds of the jurors and the judge, this too might be a tactic that Cosby's lawyers will use, the "tainted jury pool" argument.
I could be wrong, but to date I have heard that this is a 'he said, she said" case, with no third parties with first hand knowledge as to what happened between Cosby and his accuser that night, which is (unfortunately) aggravated by the fact that she waited a year to report her claims to law enforcement.