In my humble opinion
The FBI has a number of more important matters to deal with, the first of them getting on with its inquiry into Russian attempts to interfere with the 2016 US general election and the numerous allegations of contacts between Trump campaign and White House personnel and the not very subtle suggestions that they were improper or even illegal.
If the FBI has enough manpower to spend on a DDOS claim involving the FCC, which plainly was going to do what it was, they can use them instead to expedite those investigations. Whether one is for or against Trump, they are important enough to require completion as rapidly as possible.