Heathrow operates to pretty much capacity when open. That includes runway occupancy and stands. If you let too many aircraft land without letting any/many take off, then congestion on the ground can be a problem.
Also, as in this scenario when you are talking about a morning, airlines want/demand on-time depatures. Despite all the fluffy talk about wanting to save fuel and be green, the bottom line is they would rather an inbound aircraft was held in the stack (and then they would blame ATC), burning fuel, so that they could claim on their statistics that their departure times are adhered to.
This is particularly true of the 'first rotation', where to be a little bit fair to them, you need to ensure that goes smoothly so that the rest of the day runs smoothly.
The knock on effect to the days schedule if you don't get the first rotation away in a timely manner can be huge