It's not rocket science
Think of an overseas city (any one will do).
Imagine that the long-haul travel demand at that city to/from other destinations could be expressed as a ranking list, with the most popular at the top and the one-passenger-a-week at the bottom.
We're talking demand here, not the existence of air services.
You can bet that London will be top of the list at most of the world's major cities. Paris would come top mainly in the francophone Africa markets.
Now translate demand into flights and hey presto, you've got more services at London than at the other big cities.
More service equals more connecting possibilities, which means more service...