Because it's easy to install and maintain, it has millions of free templates and plugins and it's highly flexible.
Because small startups cannot AFFORD a custom coded CMS, themes, plugins or TRAINING.
Small startups/businesses in my experience are majority of wordpress users and their money is better spent on marketing, paychecks, and everything else that's needed to get the project off the ground.
I can literally set you up with a site and a ecommerce system in 20 minutes for FREE - that's why. The site that will be functional and professional looking.
The guy I work for now is doing sales of his own brand product. All his sites are in wordpress including the shop site. All of these were made by "clueless" people (in terms of IT knowledge) and he's doing $35k USD a month in sales after 1 year of doing it.
It's a small startup, and if wordpress didn't exist - his business wouldn't exist and salaries for 4 people wouldn't exist.
Wordpress has pretty good support as well. Updates are released often, and on dashboard you get notified of those updates - after which you do 2 clicks - done. Up2date, vulnerability gone. If they don't fix known bug, cool - hire somebody to fix it for you.
I know slashdot people like to shit on wordpress, PHP etc., but hundreds of thousands of businesses are on internet because of wordpress. It's a great CMS.
Internet is not only geek's playground anymore. Common people are on it now and are running their own internet business thanks to wordpress/joomla, facebook apple and the likes. But I also know average slashdot nerd don't care or understand much about how things work outside of what he does for a living.