Early, the company said that those groups would have to wait until the official Oct. 18 launch date.
[Other memorable FUBARs of late include the lack of a Start button in Windows 8 -- rectified in Windows 8.1, and the "Always On" requirement for the Xbox One, since cancelled.]
In fact, Microsoft said, it will make the RTM versions of Windows 8.1 and Windows 8.1 Pro available to those groups the same day as the announcement, meaning Sept. 9, starting at 10 a.m. PDT. Microsoft also said it would release the RTM version of Windows Server 2012 R2 to MSDN/TechNet and volume licensees on Monday.
Officials added that the Windows 8.1 Enterprise SKU will be available to MSDN/TechNet and volume licensees before the end of September. The RTM version of Windows 8.1 RT isn't going anywhere, though, and those groups will have to wait like the rest of the public. The RTM version of Windows 8.1 RT has leaked to the Internet, though.
As far as "why," Microsoft executives said they heard the complaints long and hard and took them to heart. It's unclear why they would have had to hear; common sense would say that developers would need early access to the final bits.
Of course, developers also need the RTM version of Visual Studio 2013 in order to produce to updated apps for Windows 8.1, and that is still not ready. It is close, though. The Release Candidate (RC) build of Visual Studio 2013 was released on Monday, as well.