Because without one the speculation from the public would have been rampant, and IMO, that's more cruel to the family. I don't think it was mean--they met with the family before the presser to fill them in so they would not hear about it on TV first, or let them be strung along by rumor. Much better to speak with the family in person and then release it than have them subject to wild rumors.
If my family member were missing for 15 years and the police were excavating a basement for possible evidence, I would for sure want to be there in person to know what they found, even if it was nothing, especially if it was at my family's request that they search there in the first place. I did find it interesting that there were state police cadaver dogs there in February and they did not hit on anything. I'd only heard about the private citizens' search.