Google trends is a tool that analyzes Google web searches and that can visualize search trends over time using so called "search-volume graphs". These graphs usually provide a good mechanism to compare the popularity of two or more products.
It is worth pointing out that Joomla has been around a lot longer than the graph suggests. In 2005, the bulk of Mambo's core developers left Mambo and started Joomla after a dispute with Miro Corporation, the company that founded Mambo. Keep this in mind when interpreting the graph. (I tried adding Mambo to the graph but the term Mambo isn't unique to Mambo, the content management system.)
That said, you can see that Joomla is more popular than Drupal, and that Joomla has been growing a lot faster. Why? The general consensus is that Joomla has a more appealing balance between functionality, flexibility, performance, quality of code, ease of use, documentation, user interface design, support and product marketing.
I want to add that Drupal is not trying to compete with Joomla. We are actually pretty good at ignoring the competition, and just do what we think is best to do. Regardless, there is significant overlap in functionality and many of our users ask questions about the differences between both ...