" Who in their right mind would waste all that time getting to and from airports to save such a short centre-to-centre train journey?"
IIRC it was for connecting services and access to LGW - Birmingham to Gatwick was a horrible drive pre M40 & M25 and train meant hauling your kit across the underground in London.
What killed it IMHO was the increased checkin time and security - when you could pretty mush walk on it made sense