modern approach to extensions and modularity, using composer;
simplified approach to create sub applications (I think to advanced templates) inside the same application;
simplified approach to create API resources;
simplified approach to create documentation;
improved approach to ActiveRecord and DataProvider;
So, as you, I don’t understand how people continues to support Yii 1, but I think because they have many projects based on Yii 1. This could be the only reason to continue with version 1.
I have a few Yii1 projects but now mainly use Yii2 because of… almost everything (security, speed, features). As others said, we still have a few Yii1 projects because of code itself (can’t change it now).