Skipness One Foxtrot
JetBlue indeed are pretty unknown to UK travellers so would be expected to build up predominantly with US-origin business. These passengers, more than UK ones, would look for Heathrow services, for while UK passengers likely come from a wide range of home starting points, those from the US will be far more concentrated towards Central London hotels and business meetings. Unlike some other areas of the US, from Bermuda Agreement days, those from JetBlue's nodes of New York and Boston have little familiarity with Gatwick, and certainly not with Stansted, and will be appalled with how much a taxi costs, and how long it takes, from those to Park Lane. Americans tend not to do strange trains.
Same as UK-based carriers serving Oakland instead of San Francisco, with the former being a regular failure and them transferring over.