@rozzin Regular (poor) folks cannot afford to buy a new phone every 2 years. They want updates for inexpensive phones like ~70$ Samsung Galaxy S3. So you can have fully working S3 with the latest Android version. (Also I9300 is popular because you can use Replicant ROM on it. So no binary blobs.)

Check top countries: India, China, Russia, Brazil, Indonesia and so on. Therefore affordable phones are the most popular.
Older models had time for more installs so they are higher than new one too.