I'm sure it was an issue of time and there had been repeated signals that they had to wrap up which had gone ignored to that point. Since it was unedited, there certainly was no issue of reluctance to not answer as Russert clearly tried to convey in his needless ending remark.
Russerts an asshole. The crap could have been edited out. There's no news there. Chowderhead thinks he is one of the unelected Princes of America. What an ass.