alt Hacker NewsAfter reading the story, it seems like lack of training. I think training should assume GPS goes out (which it does), in this case they didn’t pull up when they saw “lights going out” as a mountain became between them and the airfield.
Air traffic control also failed here.
Air traffic control have a very specific remit and it isn't as comprehensive as people assume. You can have "no service", "radar service" and "full service" and what they will advise you about is limited, usually, to spacing or procedural errors like "you should be less than 220 knots".
A controller who has the capacity to spot something that seems off like a wrong heading or altitude might ask the plane if they are OK, but unless they are deviating from a flight plan in controlled airspace, they are more likely to assume VFR and pilot knows best.