The issue is that a lot of other software would need python-older and python-oldest stacks. 

There is some amount of software is written for whatever languages is in the oldest active Ubuntu LTS and the developers really only look to update it to newer stuff AFTER it has gone EOL. There is another amount of software written for the latest LTS, and there is another set of software which is written for the latest. I expect that each of those groups is large enough to think they are the 'majority' of users of the languages in question.. so have no urge to move slower/faster because they really don't have the time to do so. [If you are focusing on the latest features it takes as much time and energy to slow down and find alternative methods which work in older stacks.. if you are focusing on long term stability or your main job is some other thing.. porting some script you wrote to something newer because XYZ now needs foobaz methods is not a high priority either.]