Las Vegas Not A Top International Travel Destination?

I came across this Yahoo! article London is 2015's Top Tourist Destination which lists the results of MasterCard’s 2015 Global Destination Cities Index.

I was somewhat surprised not to see Las Vegas anywhere on this list.

I'm guessing either:

Las Vegas isn't large enough to qualify as a major city.

or

This list only measures international travelers, and Las Vegas caters to mostly domestic visitors.

or

This is a MasterCard study and Las Vegas is a cash heavy town.

Also notably missing from the list is Orlando, which while undoubtedly a huge tourist destination probably suffers from the same shortcomings as Las Vegas when it comes to the criteria of this study.