Might not be on purpose, i.e. a newer os might just expect more resources to run, which are just not available on an older phone. The same happens with computer

Might not be on purpose, i.e. a newer os might just expect more resources to run, which are just not available on an older phone. The same happens with computers, try running the latest windows or macos on a 5 year old computer, it will probably run, but definitely not as smoothly as the os of the time when the computer was built.