Battery and Power Issues
In addition to eating up memory, newer software also tends to consume a more significant amount of battery life. For new Apple devices, this isn’t an issue, because Apple is consistently updating its battery technology too. But older devices can only hold and route so much power, and even replacing the battery isn’t an ideal solution when the new version of iOS needs to draw more power at a time than that older battery can handle.
The result is usually a battery that runs out incredibly fast or an iPhone that crashes all the time. Rather than let these problems get worse and worse, Apple ends compatibility for new iOS updates on aging devices.