WordPress dominates the CMS market with overwhelming numbers (61%: http://www.opensourcecms.com/general/cms-marketshare.php). I know very well the strengths and weaknesses of WP - I worked on a single WP site for 2 years, pushing its limits every day.
My general understanding is that PHP is being surpassed by Node and Rails. I'm jumping on the Node bandwagon: the synergy between JavaScript on the front and back end seems like a game-changer. Add web sockets and its unbeatable.
So maybe a better question is, why aren't there any contenders? Clearly WordPress thrives on its massive community, and maybe its just a matter of time until a worthy opponent emerges to gain the necessary momentum. I'd like to help make this happen, and I have a lot of great ideas to do so.
Why doesn't Google, Microsoft, Apple, Yahoo, etc. etc. make their own Open Source CMS / web platform that competes with WordPress? You might say they have, and I'd be interested in looking into these alternatives, yet WordPress + Joomla + Drupal have 86% market share, which is ridiculous.
Sidebar: This is my first HN post. Comparing to the heavily-moderated Stack Overflow, I'm wondering what kind of posts are NOT okay here. Let's say I have big plans to create the next Google-scale company, and need help to get started. I need to find people who can help me discuss, collaborate, design, and develop. Is HN an appropriate place to do this? If not, is there a better place? Another forum? An IRC channel?
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.