Stansted is more convenient than Luton for a significant amount of London. For example, there isn't a single traffic light, and rarely much delay, between Canary Wharf and the Stansted Terminal frontage. Even the road coaches from Victoria go out this way. In contrast the M1 to Luton can be quite unpredictable for journey time, and a tedious challenge to get to in the first place.
Way back when Go had a good selection of routes from Stansted they built up a significant following. I think quite a bit of it was London business traffic siphoned from Heathrow (a significant number of suits in the 7am departures), which set the BA rivalries going and eventually led to the demand for a sale. Easy never seemed to capitalise on this, and where I was making quite a bit of use of Go, and the early Easyjet operations, it has progressively fallen away.