I would say that people prefer other airports because they are better than Heathrow.
The idea that Heathrow is it for the UK and we must use it is clearly not being listened to by millions of passengers around the UK who no longer need/wish to use it.
The reality is there are much better airports to travel via if a direct option is not available.
Another issue that Heathrow has is the incumbent BA which is quite simply not a particularly good airline anymore.
Especially for First/Business class passengers where BA has been well and truly left behind.
That makes Heathrow less attractive because they are the biggest operator and as such passengers will fly via somewhere else to get better service. Especially East Bound.