After seeing this and this, and also adding numerous PPA's that have no logical reason to be added, I still can't get to have gnome-shell further than 3.12.2.
I am using Ubuntu GNOME LTS and I am fetching from the GNOME Project PPA
A strict answer to you question "Why?" is:
Probably you also wanted to know whether there is an easy way to install it. The answer is no. Why?
Finally, I just want to point out that your behaviour is contradictory. One uses LTSs because wants stable software (although maybe older). If you want to use new (maybe-not-so-stable-on-Ubuntu) software, you should move to the latest Ubuntu version or, even better, change distro and go for a rolling distro.
Installing the latest software (via PPAs or just by installing the debs) on a possibly older LTS release will make it more unstable than the last non-LTS.