I believe that many times the FBI is brought into a case for the reason of crimes crossing state lines and using computers to send messages regarding crimes and motive. Using telephones and computers brings FCC and federal laws and jurisdictions. Also in smaller jurisdictions crime labs are usually way behind on processing of evidence. In cases as an example of material and DNA under the fingernails of the victim. There is such a small amount of material that if it is sent to the state lab or county lab, each time it is examined the amount is used for testing plus the chain of custody gets more clouded each time it is handled.
So sending the evidence to the FBI labs in VA is more credible and faster but also the smallest amount is used for testing one time and with shortest chain of custody.
Also I think it is possible in this case the possible suspect pool may be of a younger age and no criminal activity or felonies in their background. Therefor LE might well have a bunch of DNA samples, yet no profiles to compare them to or anyone in the overall database with a match. LE cannot just go around willy nilly asking for DNA samples without probable cause. Not to mention the cost of running them has to be justified.
JMO's
So sending the evidence to the FBI labs in VA is more credible and faster but also the smallest amount is used for testing one time and with shortest chain of custody.
Also I think it is possible in this case the possible suspect pool may be of a younger age and no criminal activity or felonies in their background. Therefor LE might well have a bunch of DNA samples, yet no profiles to compare them to or anyone in the overall database with a match. LE cannot just go around willy nilly asking for DNA samples without probable cause. Not to mention the cost of running them has to be justified.
JMO's