So if we want one big bust at pretty well the same time to get everyone and avoid letting the hydra re-grow, that means quietly (very quietly) infiltrating the groups, building cases with federal-levels of inescapability, then moving.
It's not like they can even just arrest everyone on one server, either: TFA calls out how it contains both victims and monsters, and that hardly leads to an airtight case anyways
So the FBI trying to keep their investigation out of the news makes sense. It taking awhile to try to get everyone, even if they occasionally move faster for egregious acts, makes sense. Not getting much help from local forces makes sense - this pretty decidedly crosses state lines, and a county sherriff isn't likely to have anything they won't have or can't get ahold of.
Moving slowly is horrifying for the parents of the additional victims that needn't be victims. But being thorough could pay pretty rewarding dividends.
Police being unwilling to get involved in internet crime, especially harassment and abuse, has been well known for decades now. Originally when the internet was new it was "how will police respond to online crimes" and then it gradually shifted to "why won't police respond to online crimes."