The standard of proof is also different in criminal vs civil. Criminal cases are only supposed to be found guilty when the evidence is "beyond all reasonable doubt".
So, the grand jury apparently decided that there existed a reasonable doubt. Or phrased another way, the grand jury decided he was not likely to be found guilty beyond all reasonable doubt. I think each case has to be considered separately and they can't refer to each other? Not 100% sure on that one, but I thought I read that somewhere. So, unless there was a rape kit done to present as evidence, it likely becomes a he said, she said situation when looked at in a vacuum. That makes it harder for a criminal case to get to beyond all reasonable doubt and my guess as to why there was no true bill at the grand jury stage.
However, civil cases are based on a 'preponderance of evidence'. Hence why you get cases like OJ who could be found not guilty in criminal but guilty in civil. Again, I think they have to keep some separation between the cases, but it's easier to build the preponderance of evidence that Watson had/has? a mode of operation where he used instagram and the excuse of massages to get sexual favors - with some willing and some not willing participants.