Abhi WordPress is so popular because of it's ease of use, and the vast amount of plugins available for it. I've been using WordPress for years and will continue to do do. There are other CMS platforms out there - the closest being Drupal for example, but they are nowhere near as flexible as WordPress is.
In fact, I'd consider myself a WordPress guru. I have written a ton of custom functions which I still use today. Now sure about the 50% of the internet (probably 50% of the CMS market) - more like around 35%. The reason behind this success is the unparalleled versatility that allows WordPress to be deployed everywhere from small personal blogs, small business websites to powering Fortune 500s like Sony Music and the Forbes blog. Some fun stats
- WORDPRESS IS POWERING 35% OF THE WEB
- USERS MAKE 70 MILLION NEW POSTS AND 77 MILLION NEW COMMENTS EACH MONTH
- WORDPRESS GETS 487 BILLION SPAM MESSAGES EVERY MONTH
- MORE THAN 1.1 MILLION NEW REGISTERED WORDPRESS DOMAINS EVERY 6 MONTHS
- WORDPRESS IS SEARCHED FOR 2,940,000 TIMES EVERY MONTH
- WORDPRESS DOMINATES GOOGLE’S SERP WITH 2.28 BILLION RESULTS
- WORDPRESS IS, BY FAR, THE MOST USED CMS
- WORDPRESS IS AVAILABLE IN 196 LANGUAGES
- WORDPRESS IS MORE VISITED THAN TWITTER
- WORDPRESS HAS 501 TIMES FEWER EMPLOYEES THAN AMAZON
Many more facts courtesy of https://digital.com/blog/wordpress-stats/