This isn't that hard:
Bug in iOS:
1. Apple releases a patch
2. All users of supported devices can install it
Left hanging: people with old devices (minimum age approaching half a decade)
Bug in Android:
1. Google releases a patch
2. Many Nexus users can install it immediately
3. Everyone else has to beg dozens of manufacturers to ship an update for a device which brings no further revenue to the hardware manufacturer
4. At least in the U.S. everyone then has to beg carriers to ship an update to existing devices rather than using this as a chance to push you to upgrade to a $$$ new device and extended contract lock-in
Left hanging: everyone who doesn't own a recent Nexus device. Minimum age: negative – devices without the current OS will be sold to users months after release.
Note that absolutely none of this is Android's fault technically. It's only Google's fault to the extent that they naively believed everyone else would be responsible and neglected to have this license require updates, unlocking after dropping support, etc.