It’s possible the SAS and/or autopilot wasn’t functioning, in which case it could have been nearly impossible to maintain stability and control without outside references. Helicopters are not airplanes, so it’s possible it wasn’t as simple as “climb above the fog/cloud layer”. We won’t know if that’s the case until we get more information from the investigation, but it is certainly something to consider. It still doesn’t remove “pilot error” (i.e. a poor decision to fly at all in that weather) from the possible cause, though.
My purely speculative guess as to why they were flying low-level VFR is that either: the required equipment for single-pilot IFR wasn’t available on-board (either not installed, or not working), OR it was available but the pilot wanted to avoid possible delays on an IFR clearance combined with the fact that they still would have had to go visual to land off-airport. And helicopter VFR limits are much lower than for aircraft (presumably because a helicopter can just slow down and is more maneuverable than an airplane).