Why do some games only work on new versions of the iphone?

What is usually updated in the new i phones that makes it hard for games to be played on the older devices?

More RAM, Faster processor.

The new iPhones have a better and faster processor so newer games were specifically made for it. Some of the newer games are graphic intense or have a lot of things happening on the screen so a better and faster processor is needed.

It's the same reason why new games can't be played on the Atari 2600.