Is it just because it doesn't use the latest methods or languages or something? Is there really more to this than simple techno-snobbery?
FWIW I think the answer to your question is dead simple -- WordPress will still be the CMS leader in five years, running something rather greater than 20% of the web. You can't beat the entry price...
People use Wordpress because by now there's a plug-in for everything; I contribute to a smallish blog about grant writing (http://blog.seliger.com) and do simple maintenance, and I use it because everyone else does and practically everything I've ever wanted to do has had a plug-in already written.
Joel Spolsky's "Bloatware and the 80/20 Myth" http://www.joelonsoftware.com/articles/fog0000000020.html is relevant here:
A lot of software developers are seduced by the old "80/20" rule. It seems to make a lot of sense: 80% of the people use 20% of the features. So you convince yourself that you only need to implement 20% of the features, and you can still sell 80% as many copies.
Unfortunately, it's never the same 20%. Everybody uses a different set of features. In the last 10 years I have probably heard of dozens of companies who, determined not to learn from each other, tried to release "lite" word processors that only implement 20% of the features. This story is as old as the PC. Most of the time, what happens is that they give their program to a journalist to review, and the journalist reviews it by writing their review using the new word processor, and then the journalist tries to find the "word count" feature which they need because most journalists have precise word count requirements, and it's not there, because it's in the "80% that nobody uses," and the journalist ends up writing a story that attempts to claim simultaneously that lite programs are good, bloat is bad, and I can't use this damn thing 'cause it won't count my words. If I had a dollar for every time this has happened I would be very happy.
Replace "word processors" with "blogging platforms" and the same basic thing is true today of Wordpress.
It seems like unless you're willing to pay for managed wp hosting (or fancy hardware), you can't expect more than 10 pageviews/s to be handled. This makes me sad.
Caching somewhere above WordPress seems to do wonders, things like Memcache, Varnish, etc.
Also, some of the companies I was comparing are charging <$10/month and outperforming some of the bigger brand names like WPEngine (SiteGround and GoDaddy for instance). I suspect if they gain traction with their products, they could push the price down on entry level managed wordpress hosting.
This is what I think the main problem is. There are so many plugins and many (most?) are poorly written. Since most users don't know how to code beyond maybe some html/css, they just search for the functionality and install whatever matches. That can typically land you with a WP site running 10+ plugins, each with their own security / bug liabilities.
WP is fine by itself and there are lots of good plugins but like anything user managed, it gets blamed regardless of what's causing the problem.
I've used Drupal and Joomla and I can't fathom why anyone would prefer those platforms over WordPress.
Thenagain, I've been using WP since version 1.0...so maybe I have been trained to accept a way of operating that isn't necessarily ideal. But I've never heard an argument for why WordPress sucks (asides from the source code).
* You don't start interacting with it via code or text files -- you administrate it through a series of web forms. * Customization is done through plug-ins rather than composing familiar framework abstractions * If when you do decide to look under the hood, it's not organized like any familiar back end from the last 10 years and will take you longer than usual to figure out * Oh, yeah, PHP
It's pretty hard to argue with its success for people who aren't webapp engineers, though. And even from the engineering standpoint, after I had spent 2-3 years working on projects that used it as a basis for a platform, I came to a grudging (if tepid) respect for it.
Furthermore neither Statamic nor SquareSpace are opensource => vendor lockin.