Most likely those apps don't run on current versions of the OS. There's no sense downloading an app to a device where it can't run.
There's no sense upgrading an OS without maintaining backwards compatibility?
iOS is only 10 years old...
In this case, the root cause is probably the CPUs changing from 32-bit to 64-bit. There are costs of having 2 versions of all of the system libraries - maintenance dev time, RAM usage, battery life, etc.