The reason, according to Michael Atalla, group product manager in the Unified Communications group at Microsoft, is lack of resources. "We are focusing our resources on getting Exchange Server 2010, which will be fully tested and supported on Windows Server 2008 and Windows Server 2008 R2, customer ready to be released later this year."
This means that if you're planning to do a complete operating system refresh when Windows Server 2008 R2 is released later this year, you'll have to move to Exchange 2010 as well. Not that I need any more reasons to do so, anyway. Exchange 2010 rocks!