As Android evolves, you need to update drivers to keep in step with new features (pretty much the same as with Windows - you need new drivers for newer Windows usually). But as opposed to Windows, Linux kernel doesn't really have a very stable driver API and the manufacturers love to patch and mess with the kernel itself to get their hardware working. As such, porting closed-source drivers to newer Android OSes is usually impossible without support from the manufacturers.

Qualcomm (by far the most popular hardware vendor for Android devices) doesn't care about supporting their released hardware.