Because the ultimate goal is to have you on the most secure OS the manufacturer is supporting for your device. Face it, whatever OS you're running now WILL have holes in it that could be exploited under the right circumstances, any newer OS will have fewer, or at least have the longer-known and more likely exploited holes patched.
That said, there is absolutely NOTHING you can move off to make space? Even temporarily? You could make sure you have your full-resolution photos backed up and remove them. How much space do you need to free up?