Why not earlier? In large part it's because Microsoft is making the Windows sausage differently these days.
We are no longer in the world where the Windows team has three years to plan, build, test, modify and then RTM a new version of Windows. Instead, since Windows 8, we've entered a world where Microsoft is giving itself about a year to perform all of those tasks.
Microsoft doesn't need to try to perfect Windows as much as possible before declaring it ready to RTM. The company can RTM Windows and then push patches, fixes and updates to it right up to the time it is generally available -- and continue on a regular basis after that. Supposedly, according to my sources, there will be at least one big batch of updates for Windows 8.1 and the Microsoft-built and bundled apps (like Mail, Xbox Music, the Bing apps, etc.) pushed out shortly before general availability.
Because of OEM requirements, Microsoft still needs to get PC makers the RTM bits a couple of months before they are expected to deliver new devices with those bits preinstalled. That's why Microsoft is RTMing Windows 8.1 in late August; that will give OEMs a couple months to test, optimize, add crapware (sadly), preload and package up those machines.
As things are different now, it's worth asking whether Microsoft will continue its recent practice of launching new flavors of Windows Server and Visual Studio in conjunction with each new release of Windows. Will Windows Server 2012 R2 and Visual Studio 2013 (which recently went to preview) also launch on October 18? The answer, for now, is yes, re: Windows Server, System Center 2012 R2 (and the next version of Windows Intune). No word yet on VS 2013.
Update: While the Windows team isn't confirming that MSDN/TechNet won't get the RTM bits until October 18, the server team is saying this is the case. I asked whether any customers would get the Windows Server 2012 R2, System Center 2012 R2 or new Intune bits before October 18 and was told no by a spokesperson. "Customers eligible to download that day are those who have active MSDN or TechNet subscriptions, and those customers that have access to future versions based on their VL use rights will get the products via VLSC downloads" starting October 18, the spokesperson said, via e-mail.
It will be interesting to see if Microsoft continues to keep Windows client and Windows Server in lockstep once the new sausage factory (a k a the reorg'd Microsoft) takes hold. There's no reason these have to continue to be aligned launch-date-wise. As the Office team already has learned, fewer arbitrary ship schedules need aligning when going cloud-first (which is what Windows Server is now doing)....