Earlier this year I visited the NTSB's lab for recovering data from "black boxes" and other data recording devices on aircraft and other vehicles.
I remember my host said that NTSB is mandated to investigate every aircraft accident in their jurisdiction, but they can pick and choose which accidents to investigate in other modes. Also, for many nations that don't have the resources or expertise for an aircraft accident investigation, NTSB does the work pro bono.
Most aircraft accidents are general aviation (not commercial airliners) so it's a similar situation where a lot of NTSB resources are spent on what is largely a rich-people pursuit.