Software
The reason for this is down to how Microsoft is pushing this new OS into the market. It's focus is purely on the software, not the hardware. So most people will be upgrading from Windows 7/8/8,1 to 10 on the existing, perfectly serviceable, hardware. Coupled with the free upgrade there is no need to stump up any cash to get the latest OS, just an hour waiting for the on the fly upgrade - which, I have to say, worked fine on a couple of our older Win7 PCs.
It's up to the hardware manufacturers to release their kit with Win10 directly installed on it. If the rolling updates continues form Microsoft then Windows 8 will be the last time the distributors will have ever stocked up for a new Windows version ever again.