If you want a bigger heathrow vote for a party which promises one, if more people vote the opposite way then it will not get built. Very easy concept to understand even for patronising keyboard warriors who bemoan that others cannot understand their grand concept.
Democracy in practice does not work like that. Only a handful of constituencies matter, the remainder are tribal (Milliband
s, Benn
s, Sarwar
s in way that would disgrace a bannan republic), and often have the equivalent of a monkey with a rosette elected. These are safe seats. The seats in play get 90%+ of the attention, as swing seats decide elections. Given the number fo swing seats near LHR, parties come out with any old garbage just to get someone elected so they can form a government and do 25% of all the other stuff they think needs doing.
People being people, they would like all the benefits of LHR with none of the costs, hence they vote for growth "elsewhere" and some blonde idiot suggests a floating island offshore costing billions
outside his electoral area. Hence a democratic system slowly over time destroys a strategic national asset for the sake of NIMBYs.
No one can vote for an expanded LHR because people won't vote for it. That doesn't mean it shouldn't be done ASAP, it just means localism is put before national interest in order to get elected at all.