Why Drupal?
When choosing a platform for business-critical systems, it's probably not a good idea to go with the "flavour of the month" solution. Drupal is mature yet evolving and has a rapidly growing user base. Worldwide, the number of live Drupal installations has more than doubled in the past year, and is currently around half a million. Growth continues at around 7% month on month (source: drupal.org).
Drupal has been attracting a lot of attention as a platform for business oriented web solutions. For example, it was recently nominated as one of three contenders for Best Business Application by LinuxWorld. Linux + Drupal is a very serious alternative to commercially available systems.
Another indicator is the recent launch of acquia - a company specialising in commercial use of Drupal, co-founded by Drupal's original creator Dries Buytaert, and which attracted $7 million investment capital.
Some time ago, a team from IBM evaluated various leading content management systems and Drupal came out on top. Their criteria included ease of installation, learning curve, extensibility, scalability and adherence to web standards. See the article here for further details. Since that time, Drupal has evolved considerably with thousands of add-on modules available and hundreds of thousands of sites worldwide using the system. To read about a selection of them, check the success stories page on drupal.org.
Economics - Drupal is a very good solution for many common web applications and it's open source - no licence fee. Businesses are always on the lookout for new solutions to reduce costs and increase efficiency, but they are especially interested when times are tough. But a zero licence fee is only part of the story - Drupal facilitates a RAD approach, offering fast prototyping and reduced development costs.
And more reasons - some articles elsewhere on the subject:
Drupal success stories and main site