Pardon my ignorance, but... doesn't Android run on top of the firmware? You're saying newer versions of Android are breaking backward compatibility with older firmware?

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.

Its a bit more complicated than that.

Quite often these days when a part is initialized, the driver uploads firmware as part of that.

If you look at most Linux distros they have a package of firmwares that various companies have allowed to be distributed with the kernel.

Meaning there is a small part of the Qualcomm blob that needs to interface with the kernel to upload the firmware.

And the interface changes as the kernel changes. This because Linux devs don't want the hassle of trying to debug a kernel panic only to find that it is happening inside a third party blob, and thus they can't fix it. Instead they want companies to either work with them to maintain drivers or hand them the information needed to maintain a open driver within the kernel source.