Under such circumstances as is the case here, the reluctance to call Mayday to ATC is more of a belief that "it can't really be happening to me". The failure of the other onboard pilots to not insist on a fuel stopover, or to have carried more fuel in the first place, is also a case of not being realistic of the situation they were putting themselves.
Having listened to the ATC recordings, incredibly well done to the Lady Controller. She remained calm, focussed and very professional throughout.
A technical failure is often very easy to declare "PAN" or "MAYDAY", if however the severity of the situation is career threatening then clearly some people might try to avoid the inevitable by keeping quiet until the last minute, or sadly until it is too late.
J