To suggest that Manchester airport is not easy to access is nonsense. In the same way getting to Heathrow can be very easy but for many also involves a bit of a trek. They have pluses and minuses in that regard.
I don't think people have an issue with Heathrow getting another runway but they do when it comes to potentially billions of public money being spent around it.
Heathrow is an asset but let's not kid ourselves that it's the be all and end all. It isn't.
Airports like Manchester offer an alternative for millions of passengers and rightly so. That will only grow year after year and the owners of Heathrow know it. They are a business and they want to keep their market share so anything they can do to secure that is understandable.
However it should not be given any preference over any other airport just because it's Heathrow and in London.