I've long thought that Gatwick has much to offer but was held back during its BAA days, the poor cousin to Heathrow. Whilst it will continue to live in the shadow of Heathrow whilst many airlines decide Heathrow is the be all & end all for them, at least Gatwick now has its own, very positive & strong voice.
LHR is the "be all & end all" for carriers because (1) that's where the premium business wants to be; (2) its hub status provides a wealth of connections with through ticketing (as opposed to self-connect which is mainly the case at LGW); and (3) the clustering effect that carriers (as is the case with all businesses)
want to be where their competitors are.
It is not a criticism of LGW, far from it. The two airports have different roles and perform them well. It is pointless for LGW to attempt to be a mini-LHR in an attempt to obtain permission for another rwy. A second rwy at LGW or not should be assessed on the airport's own merits of which there are plenty.