Running an Exchange server is a costly and complicated affair for businesses. Why spend the time and money when Google Apps is affordable, easy and available to your employees from anywhere they can get to the web even without Outlook?
Account lock-in is a skill Microsoft learned well from their once partner IBM. Having all you employees and years of data on Exchange are a strong argument for leaving well enough alone, and staying on Exchange. Moving to Google was always a manual affair. Until yesterday when Google announced a migration tool from Exchange to Google Apps. I bet a lot of companies will seize on this.