First things first, Drupal just isn't a very good product. I've spent the better part of the last five years working on Drupal. Not on the core patches (I'm no good at those) but on the other side of the technology, the part the people use. And I'll tell you for certain, while people very much want websites, they don't want just Drupal. Even if they thought they did, they won't soon after they log in for the first time with UID 1.
Wordpress and Joomla solve real-world problems, they make blogging and simple content management relatively easy. The code may be relatively brittle, the API unworkable, but when you turn them on they do something obviously useful. Can we say that about Drupal?
Of course, this failing hasn't mattered much to Drupal's growth so far. The fundamental metrics of Drupal's health as an open source project -- core patches, community members, deployed sites -- are still on an unbelievable exponential growth curve. For Drupal firms, it is still sellers market, there are more demand for Drupal services than qualified supply. Can Wordpress and Joomla say that? Not exactly.
The internet is comprised of more than more than two hundred million websites with the top 50 sites claiming 40% of the total internet traffic and the long tail of sites are scattered across thousands of commodity hosting service providers. Most of these websites are rather complex, all of them are unique.
I don't know if a Joomla, or a Wordpress, or Drupal will ever run a significant percentage of the internet's websites, but I do know that most websites require functionality beyond blogging and simple content management. A website can serve any digital purpose, that's the beauty of the web's organic and distributed invention. But this freedom brings inherit complexity for technology projects endeavoring to power a great many websites. This is a hard problem. It's the kind of problem that requires thousands of engineers working collaboratively for a number of years to solve.
While Drupal's success as a product so far is limited, it's success as a tool web developers use to build websites is remarkable.