TC explains the reasons behind this acquisition and to me the crux is:
At its peak, FC had 4 million unique monthly visitors. Since FC focuses on the negative news coming out of startups, and TechCrunch tends to focus on the positive, this combination may seem odd. But the sites are in fact extremely complimentary. For example, the audiences are about equal in size and have very little overlap. So from day one we will double our reach and traffic.