Wordpress dominates, I think, for a number of reasons. Primarily, low technical debt for users - it's as close to a plug and play application as i've seen, and nowhere are you expected to understand 'arcane' concepts like version control or the terminal. Want a plugin? Done entirely through the web GUI. Want to upgrade? Same thing. New theme? Same thing. The vast community also helps, obviously, but that community is only as big as it is because the barrier for entry to setting up and modifying a Wordpress install (assuming all you want is to use existing plugins and themes of course) is so low.
Also, many existing webhosting accounts come with an option to install Wordpress on the account. Wordpress even has its own self-hosted service. Many of the alternatives i've seen, in terms of frameworks and CMS software, haven't been able to replicate the relative painlessness of Wordpress' model.
A contender would, first, have to deal with the network effect of Wordpress which is considerable, but also (if it's written in a language other than PHP) the network effect of PHP. Since the UX in Wordpress generally works pretty well, switching to another CMS would be a difficult sell to the general user (why switch from something that's easy and works and you can hit a developer for by throwing a rock out of the window?) meanwhile most of what is terrible about Wordpress is only apparent to coders, who frankly don't really matter in this dicussion, since they're probably using Octopress or something anyway.
I think this is the crux of it. All the objections to WordPress are from a developer's point of view. But when you scoot around to the user's side of the table, all of those issues are invisible. WordPress dominates because WordPress addresses its users' needs well enough that it's really, really hard to be so much better that you can overcome the network effect. In order to beat WordPress, you need not just a better technical foundation or some whizbang design — you need something that makes it so much more attractive that people who aren't developing for the platform say, "You know, this is worth the extra effort compared to WordPress."
Also, many existing webhosting accounts come with an option to install Wordpress on the account. Wordpress even has its own self-hosted service. Many of the alternatives i've seen, in terms of frameworks and CMS software, haven't been able to replicate the relative painlessness of Wordpress' model.
A contender would, first, have to deal with the network effect of Wordpress which is considerable, but also (if it's written in a language other than PHP) the network effect of PHP. Since the UX in Wordpress generally works pretty well, switching to another CMS would be a difficult sell to the general user (why switch from something that's easy and works and you can hit a developer for by throwing a rock out of the window?) meanwhile most of what is terrible about Wordpress is only apparent to coders, who frankly don't really matter in this dicussion, since they're probably using Octopress or something anyway.