First, welcome Tim McWrath who took over PAMBA's PostNuke website when I left. Sorry Tim. Tim wants to know why Drupal when there are so many other CMSes out there.
1) Solid module architecture for adding features
I tied to create a public member database in PostNuke that would indicate whether someone was a member or not. I struggled for weeks. Creating modules for Drupal is MUCH easier and because of this...
2) Many more modules exist
Because it's easier to create, almost everything you'd want to do someone has started. I created a module called Pomise that allows users to commit to doing something if x number of other users do it. The membership functionality already exists in modules like CivicCRM as well as many others. Almost everything I've thought about doing, someone else has already done or at least started because...
3) The community is VERY active
I recently presented at the Yahoo! Open Source CMS Summit. There were groups there from Joomla!, Plone, Alfresco, Midgard, and about every other open source CMS out there. Drupal tended to dominate the sessions because it dominates the non-profit, community building, social networking space. Another reason Drupal tends to dominate are the key players are REALLY smart. People like Ken Rickards who developed the MySite module as part of his bigger Drupal driven site SavannahNow. The guys and girls from Bryght and Lullabot who build sites like NetSquared and Partipate.net. Or the folks from CivicSpace Labs who developed the software used by grassroot groups who supported Howard Dean. That project created a preconfigured Drupal install that is now much easier to do with...
4) Install profiles
I had read about install profiles, but it wasn't until I saw a demo at OSCMS that I realized that if a groups of the more geeky IMBA members got together we could create something every club could use.
I've looked at other open source and commercial CMSes and I really like Drupal. I'm biased now because I've really dug into it with OurTahoe.org, but even as I dig deeper and deeper into the code and the community I only like it more. That was not the case with PostNuke. I liked it on the surface, but when I dug into the code it was a mess. When I tried to find help in the PostNuke community, I came up empty.
I'm willing to look at other CMSes. Jason suggested Magnolia which looks nice, but requires Ruby on Rails. Ruby is cool and new, but not available from many hosting providers. Northwestern's J-School is using CMS400.Net from Ektron which isn't open, free, or as feature rich as Drupal from what I've seen. The advantage of going with Ektron or Magnolia is there is commercial support. With Drupal, you have to rely on the community. Since that's how most clubs use the 'rely on the community' mantra, I think Drupal is a good fit for IMBA clubs.