Probably because RIM are incredibly late on delivering SBS2008 versions of BES/BPS?
Wouldn't know, I don't touch SBS with a bargepole after many sleepless nights of frustration with it in the past, its a false economy IMHO. Multiple eggs, single basket.
and haven't even considered Exchange2010?
BlackBerry - BlackBerry Enterprise Server support for Microsoft Exchange Server 2010
BlackBerry® Enterprise Server v5.0 with Service Pack 1 and Maintenance Release 1 is now fully certified with Microsoft® Exchange 2010 and BlackBerry® Technical Support Services are readily available.
All existing BlackBerry Enterprise Server customers running v5.0 are eligible for this maintenance release at no cost.