In some cases you may not see the online Microsoft services as a viable option simply because you may have other pieces in play within your organization that are designed specifically for your environment, such as automated processes that you don't see moving over to the online services easily. In global enterprises, these factors weigh heavily against change.
And some folks simply don't feel comfortable with trusting others with their data, especially when they start considering compliance issues, legal concerns, and so forth. And there are licensing issues that make it hard for some organizations to move to Microsoft's hosted services.
With a modicum of investigation, however, you might just find all of your excuses not to go to the cloud vanish in a puff of smoke.
What are your plans? Have you used BPOS or are you planning on moving to an online solution (Microsoft, Google, or other) in the near future to offload some of your on-premise infrastructure and personnel needs?