Finances and investment... not so much that their corporate HQ is legally in the city of london specifically but the people arranging debt/financing to do big costly telecom projects will be adjacent to the NYC and/or London financial markets. Same reason why a lot of submarine telecom projects are closely linked to things in London. If you need to spend a ridiculous amount of money for something that has a 10, 15+ year ROI you need a massive amount of debt and that money can only come from a few sources in the English-speaking world.
Taxes, US regulation, military shenanigans?