and not one ppa, which fits for every ubuntu version and all the following versions ?
A global PPA would be a nice idea.
I'm not sure what you mean. A single PPA has pockets for every release, so you just have to add the one PPA and apt will get the appropriate package for your release.
As for why there are different packages for each release, that is because the package depends on the libraries in the release for which it was built and may not work on another release.
I guess it would be confusing if the PPA only had software for certain versions. You add it, nothing happens. Also, if you had global PPAs, how many versions of Ubuntu would they have to support? How fast after a new version would they have to add support?
It's a nice idea, but probably impractical.