The reason is the same one as in the case of Windows XP SP3: a compatibility issue between Microsoft Dynamics RMS and both SP3 and Vista SP1. Although Microsoft hasn't been too detailed in outlining the nature of the bugs, the issue had been outlined as early as April 24th on the RMS support forum.
The Microsoft Dynamics Retail Management System (RMS) Development team has identified problems when Windows Vista Service Pack 1 (SP1) is installed. Windows Vista (SP1) was released on 3/18/08 and may cause data loss and corruption in Microsoft Dynamics RMS databases.Makes you wonder if there are any other compatibility issues, doesn't it? We already know about issues with SP1 and certain "older drivers."
At any rate, Microsoft is working on filters to prevent distribution of the service packs to those with RMS installed - just as it has already done with SP1 and the problematic drivers - so distribution of both service packs should resume soon.
Just when the actual bugs will be fixed is still up in the air, however.