Istanbul should still appear, because its second airport, Sabiha Gökçen International Airport, is on the Asian side and handled 29.6 million passengers in 2016. Its airport on the European side, Istanbul Atatürk Airport, handled 60.1 million passengers in 2016.
Yes, huge cities without much air traffic. Kolkata and Dhaka also have more than 10 million people in their metro areas, but very few of them use air travel as a regular travel option.
Iran as a whole is pretty isolated from the outside world, and how many people do you know who have been to Kolkata/Dhaka for a holiday? Neither are the most popular tourist destinations.
Tehran has plenty of air traffic, but it's split between two airports. The two combined have 25M passengers a year, more than enough to be on the list.
- Taoyuan: 35.4 m
- Osaka: 25 m
- Hyderabad: 25 m
- Denpasar: 21.5 m
- Chennai: 21.2 m
- Kolkata: 19.8 m