I have always tried to use wordpress just because that's what I'm comfortable with. However it really depends on what you're looking to accomplish on your website. Each CMS has it's strengths and flaws. Personally if you really want to quickly find plug-ins and work on developing I believe wordpress is great for that for people starting out. I've always preferred going a custom route though, that's a bit more expensive obviously, but always good to control everything if very serious about a project you're working on for a long-term avenue.
I think it's obvious that the most popular seems to be Wordpress...
I think it has a lot to do with how long Wordpress has been out and how simple it is to plug php and other wordpress embed code into it...
I make own CMS, because 3rd party always miss features I want to have, or have to pay for those.
Or have too many features I don't need, which make my websites really slow.
Basically Wordpress, it powers 25% of the web and it's community is so vast that it almost single-handedly holds up the entire PHP ecosystem. Also, it's fairly easy for people to use.
And ignore what I said about Wordpress, the thing looks to be fairly insecure.
It would seem that it uses some variant of sprintf to generate a query string which is in turn executed rather than using real prepared statements and that's before you get into the ecosystem.
According to multiple kinds of research and rates, WordPress has the leading position, with Joomla following it shortly after. I believe that if you want a reliable and secure solution, then it is wiser to choose WordPress.
I like Wordpress and Joomla! the most. I've used both, but Wordpress is much user friendly IMO. Many of my favorite sites use Wordpress for their main website, and it works well.