Imagine that you want to pay someone to firstly create a 5 page website, secondly make it have an original web design and finally have it hooked up to a blog script (eg.
Wordpress,
Textpattern,
b2evolution,
Datalife Engine,
Umbraco,
DotNetNuke) or a CMS (eg.
Typo3,
Drupal,
Processwire,
Modx,
Symfony,
Joomla,
Expression Engine, etc). How much would that cost?
A british web (design and) development they would charge £40,000 for the entire project. That's $50,546 in US Dollars. Myself as a british freelancer as a sole individual, I would charge £400 for that. But with globalisation, with the inflation, cost of living, average wage and living standards varying in different countries, people in foreign countries can undercut british citizens like me to charge even lower prices.
Typically running a website requires some sort of programming knowledge but as most people aren't really programmers, they'll pay someone else to do all the web development for them, on an ongoing or ad-hoc basis.
Not everyone is keen on dealing with the prospect of programming, so turnkey scripts started to appear, the most famous one is Wordpress that powers 42% of the internet. Yes it does have some CMS type features like custom fields, custom taxonomy, REST API, etc, but that is there as an afterthought when really Wordpress is primarily designed for blogs. It's not really a CMS like the rest of the CMS scripts are.
Well anyway, the reason why Wordpress is the most popular is for two reasons.
First because it's extensible instead of bloated (bloatware) as right now there's 59,318 plugins, whereas in the past CMS scripts like
Xoops, PHP Nuke and Bitweaver (the last 2 are dead) tried to do everything at once, which made the script bloated. Note that a CMS or blog script having more plugins, does not automatically make it better. For example, Processwire only has 572 plugins in comparison to Wordpress' 59,318, not because Processwire is worse but because Processwire is INTRINSICALLY extensible, as to create an image gallery in Processwire its architecture lets you do it in 2 minutes without needing a plugin, whereas to do so without a plugin on Wordpress is a painstaking uphill struggle.
And finally the second reason why Wordpress is very popular, is because it's turnkey. Using Wordpress doesn't require any programming or web development knowledge. You just type in a blog post with the title and content, choose a category and tags, then click save then it's published online. There's no need for opening Photoshop or dealing with HTML and CSS in a code editor or IDE (intregated development environment), as you can choose one of the millions of themes online, that are ready-made to work with Wordpress without any additional configuration.