You could not have created adequate transport infrastructure needed from scratch for a venue that size in the middle of nowhere without eye watering investment. One rail hub, compared to London with rail connections to every corner of the country terminating at one of Euston, Kings Cross, Paddington, Waterloo, Victoria and others, with local distribution across a subway network.
I once inadvertently got caught in traffic coming out of the NEC when crufts was on and got stuck for 2 hours.
Not to mention the fact that people enjoy going to London for a showpiece event. Eating, drinking, hotel options are plentiful. Then there is the football heritage argument.
If they had built a national stadium by the NEC people would have hated it.