My guess is that Apple writes the drivers themselves. That makes sense because iOS isn't like Linux or Windows where there are public resources on writing drivers. Because they write the drivers, it's very easy to port over to the next iOS version because all the expertise is already at Apple.
In contrast, Qualcomm gives the OEMs a copy the custom Linux kernel that works with their SoC[1]. The OEMs then use that source to build their kernel, probably making minimal changes to it. Once Qualcomm stops selling that SoC, they stop releasing updates to their custom kernel because there's no pressure from OEMs to keep it patched.
Microsoft bundles drivers for most of the popular hardware. I was able to stick a Windows 7 DVD in my Core Duo Mac Mini, install it and it recognized all of my hardware - FireWire, Ethernet, sound, WiFi, Bluetooth and it sort of recognized my IR port used for my Apple Remote.
In contrast, Qualcomm gives the OEMs a copy the custom Linux kernel that works with their SoC[1]. The OEMs then use that source to build their kernel, probably making minimal changes to it. Once Qualcomm stops selling that SoC, they stop releasing updates to their custom kernel because there's no pressure from OEMs to keep it patched.
[1] eg. https://github.com/MiCode/Xiaomi_Kernel_OpenSource/tree/ceph...