Only Con is retroactive (i.e., for hit points) all other abilites are not.
There are several problems with making Int retroactive:
How about spells? A starting wizard gets bonus spells based on his Int modifier.
Skills cost different depending on whether or not they are cross-class or class skills. If Int is retroactive for skill points, then how does one spend them? At the current class' rate? - not good since skill points can't be saved between levels.
How about starting languages? A character gets a number based on his Int mod.
There are more I'm sure.
There are several problems with making Int retroactive:
How about spells? A starting wizard gets bonus spells based on his Int modifier.
Skills cost different depending on whether or not they are cross-class or class skills. If Int is retroactive for skill points, then how does one spend them? At the current class' rate? - not good since skill points can't be saved between levels.
How about starting languages? A character gets a number based on his Int mod.
There are more I'm sure.