Hint. it's most about networking to the rest of the world, hence "global city".
And how are Los Angeles, Chicago, DC, New Delhi, Houston, Osaka, and Jerusalem less well connected than Madrid, Warsaw, Dublin, Vienna, Budapest, Athens, Prague, and Barcelona? Houston is the busiest port city for incoming international shipments for the largest consumer of goods in the world. Pretty much every major exporter has offices here. Here's one where a rather direct comparison can be made. Dallas and Houston share a very similar relationship to that of Barcelona and Madrid. One of each pair is the most significant regional entry point, one hosts a great deal of major corporate headquarters (Madrid - 3 of the top 100, DFW and Houston each host 25 Fortune 500 companies, combined more than NYC with two thirds the population), relatively similar populations (DFW 6.3m, Houston 5.7m, Madrid 5.8m, Barcelona 4.9m). DFW and Houston each have GMPs of around $390bln. The combined regional GRP of Madrid and Barcelona was $344bln in 2004, with the DFW and Houston numbers coming from 2007. I'll give them the benefit of the doubt and say that currently the GMP/GRP of Houston, DFW, and Barcelona/Madrid is somewhere north of $400bln. So not only do both DFW and Houston each have similar economies to that of Madrid and Barcelona combined, they're each far more important as centers of international business than both Spanish cities. You can talk all you want to about history and culture, but such an overwhelming difference in financial strength and influence is far more important than what is represented by the list.
DC and New Delhi are self explanatory. Chicago has been home to the busiest international airport for all but about 10 of the years that airports have existed; networking doesn't get any more direct than that. LA is a blend of Houston and Chicago, along with hosting the most preeminent global entertainment industry. Jerusalem has all of the historical importance of other major religious centers, such as Rome and Istanbul, while currently playing a much greater role in the public eye than either of those cities. Osaka is the second or third busiest port on the planet, the fifth biggest metropolitan economy, and claims the historical Japanese cultural center of Kyoto as part of its metropolis. Japan may be more isolationist than much of the world, and Osaka may be overshadowed by Tokyo, but two thirds of the European cities which precede Osaka in that list have
no appreciable direct international trade outside of tourism and human services.
They seem to have built that list off of connections between cities of different nations, which would certainly explain the overabundance of European cities early on(If you were to treat the EU as a single nation, there would still be a slight European flavor to the list, but not the overwhelming bias shown here; London would certainly no longer precede New York, Tokyo, Singapore, Beijing and Shanghai), although the ridiculous number of British cities is only explicable as biased results.
