Why handset manufacturers releases new versions of android several months after Google?
It may help to go through this graphic:

Notice that the graphic has a blue 'Google Play Edition' path which was when non-Nexus manufacturer had devices with only stock Android. This blue line would be the closest approximation to a Nexus device and notice how it doesn't have as many blocking points as the orange 'Carrier Device'.

Comparing to PC

You have to realize that the PC Hardware layer is relatively standardized on a x64 architecture either from Intel or AMD.

Android has multiple CPU architectures to support (ARMv7,ARMv8,x86,x64,MIPS) manufactured by different vendors (Samsung, Qualcomm, NVidia, Rockchip, Intel, MIPS) and its in Step 5 that the various vendors have to see what will get support. Way more customization goes on here than a normal PC.



Originally from:
http://www.htc.com/us/go/htc-software-updates-process/