Dave -
Because LHR is better situated within the UK.
Just take a look at the geography. Gatwick road and rail connections are virtually non-existent east-west. Go south 30 miles and you're in the sea. Go north and you hit Congestion Central.
From LHR to the west there is a good motorway to Reading, Swindon, Bristol, Exeter, Cardiff, etc. To the southwest there is good motorway into Surrey, Hampshire, Southampton, etc. Northwest is Chilterns, Oxford, Birmingham by quiet motorway. Within 20 miles north you are on the M1 towards Watford, Luton, MK, Northampton etc. Rail links into Central London are many and quick, and offer a wide variety of destinations.
Forget the hubbing and spoking - it's location, location, location.