They don't have a monopoly, but as soon as it comes to creating a website, it's the first tool that gets mentioned when one starts talking about a CMS.
It's simple to install, simple to use, and has a vast array of free themes... And yet, I refuse to use it, neither as a managed solution nor as a self-hosted one.
Is it the best tool?
It's IT, so the answer is: it depends.
For what I want to do here, it can certainly handle it.
Does it fully meet my specifications?
Not entirely (the fact that it relies on a database already makes it too heavy for my use).
Just to differentiate myself from the crowd and attempt to assert some semblance of uniqueness, I prefer to turn to another solution.
My choice is detailed on this page of the wiki.