WordPress is popular for lots of reason here I’m point out some of the main reasons:
1. WordPress is very easy to setup and use. It is very easy to customize and edit, being open source
2. You can create Any type of website with WordPress like- An eCommerce Website, A Membership Website, Directory or Classified website, Service/Appointment Booking Website, Question Answer Forums, Blog etc.
3. WordPress is constantly updating and providing bug fixes and new features frequently
4. The price of wordpress pre-made themes is lower than any other CMS
5. WordPress Is 99 % SEO Friendly [Acc. to Google]