It's quite simple really, non of the airlines you have quoted are based in London, to operate on MF you need to be within a fairly close proximity of London - the average wage in London is £32,604 per annum and £26,728 in the South East.
Is Gatwick not close to London? Is Luton not close to London? Crew often make the false assumption that Heathrow is in London and crew working from Heathrow must live in London. They don't, and there are plenty of affordable places to live within an hours comfortable drive of Heathrow.