Essentially this question is asking why cities such as London and New York are such prominent cities in respect to the rest of the world.
Ask this question to yourself - Why don't other cities (e.g. Zurich, Oslo, Manila) have such a strong economic and cultural influence compared to London and New York.
You would then say it is because _____________________________.
For example mention things like:
- TNCs
- Deregulation and liberalisation
- Communication and presence of many different cultures under the guise of one city
- Governance