Open source solutions' upgrades are expensive or infrequent.
Today’s Web businesses need to constantly evolve to survive. While open source technology is continually changing, choosing and maintaining a single distribution is a time consuming and risky business, especially since open source APIs often change with each rev. In-house teams focused on maintenance can miss technological advances, while constantly turning to agencies for support quickly becomes astronomically expensive.
Drupal has a very solid, public roadmap around major revisions, and best practices for upgrading between major releases. In addition, the very nature of open source software and its communities fosters innovation. They are often far ahead of proprietary competitors in providing cutting-edge technological advances into the very latest technologies. For example, Drupal was the first CMS to offer RSS feeds by default and Drupal 7 will be the first to offer RDFa as a core technology.