Apps are rarely self-contained you build on layers all the way down to the OS and BIOS/Firmware, I can fire up an old windows or DOS program only because Microsoft still support ancient API's or the BIOS API's still exist on the PC. The older phone processors are still perfectly fine but Apple and Google are not going to support them as they may be slow with new (bloated) code or may not have some fancy feature they use for one tiny part of their latest OS, same with the frameworks that build on top of them and with everything web these days there are a lot of layers.
