No doubt that today (2014) this is true, but why? I will compare it with another CMS that I know, Joomla. In mid-2000, Joomla was the king, I installed dozens of them at that time. But today Wordpress is almost the only customers demand. I've read a lot about the subject, and the first thing that is true: As CMS, Joomla is better. That is questionless: Native language management, better scalability (plugins, modules and components), user management and permission roles, etc. too many things. The fact is that Joomla born as CMS, while Wordpress born as a tool for bloggers.
I have read authentic falsehoods about why Wordpress is now the most used; like it has better templates, it is better for SEO, is less hacked, etc. all that is false, any experienced Joomla user can easily prove it. But it is true that today, Wordpress has the best authors of plugins and templates working for it; is no longer a tool for bloggers, but an authentic creator of websites. How did we get here?
The fact that Wordpress is the king proves that what people want is to write content, they want a system as easy as possible for it, and the admin of Wordpress has been always the easiest to use, because once installed, it has the precise options to start writing content, not have dozens of options like Joomla, which makes Wordpress easier to explain and use for people with less computer knowledge.
The blogger community, the most content creators in Internet, are who have boosted Wordpress. Google has a part of this too, since they promote the creation of content, either with good or bad intentions (remember these "farms" of blogs for link building and other “black hat” SEO practices). So if the user community is triggered, it also triggers the developer community, no matter if the CMS is the best or not. The case is reminiscent of the VHS and Beta video affair, though obviously in different circumstances.
Another element that clinched Wordpress is the huge number of free templates and plugins. But of course, Joomla also has free plugins and templates. I think the issue was that matched the time when many components, modules and plugins for Joomla began to be paid, when most Wordpress plugins remained free. That boosted Wordpress even more. Today many of those plugins for Wordpress are paid too, but it does not matter, because Wordpress is the most used.
We could also establish a similar analogy with the simplicity and cleanliness of the code: The era of Wordpress simplicity and cleanliness of code matched when Joomla components started to be very complex. Today Wordpress has the same or more complexity, there are framework layers, etc ...
Because when you are the dominant one, you can do whatever you want. Why Wordpress is not multi-language without plugins? While plugins like WPML do a good job, it still not enough. Wordpress should have a multilanguage system once installed, now we need different tools for translating interface (Poedit) and content (plugins like WPML). I think this disinterest is heritage of blog community, as they write their content in their own language and rarely translate it to others.
Do not forget that Wordpress serves to the content, not to the interface. Therefore all new features of Wordpress in these years have gone in the direction of improving the writing of content: Best tool for uploading multimedia, best visual editor, the shortcodes, etc.
Nowadays Wordpress can do little to improve as CMS. Sure the authors would love to rewrite the code in order to add all those features that we miss (or maybe not), but when you have millions of users, thousands of templates and plugins, dozens of frameworks, it is too late to change things from scratch.