Limited Memory Creates Issues for New Software
We’ve probably all encountered apps that stop functioning because they weren’t compatible with a recent software update, forcing people to roll back the update or wait for the app to come out with a patch of its own (or sometimes, just deleting it altogether). Apple encounters this problem as well, but with its own software. Specifically, memory issues can make it difficult to keep supporting older devices.
Remember, Apple is constantly updating iOS with new features and with greater efficiency. Every iOS increases efficiency a little more, but it can only do so much with existing hardware.
Eventually, iPhone and iPad devices reach a state when they can no longer keep up with all of the features that iOS supports due to a lack of memory, and so they cannot be made any more efficient through software updates.
Apple has little incentive to make alternative versions of iOS that are missing important functions just so that they’ll work on older devices. So instead, Apple cuts off support for devices that no longer have the memory to run its software. The more complex iOS becomes, the more devices are cut off, usually with a timeframe of 5 to 6 years or so.