I am using Ubuntu GNOME LTS and I am fetching from the GNOME Project PPA
A strict answer to you question "Why?" is:
- Because the maintainers of the repositories decided to not port it to 14.04, as you can see when filtering the versions of Ubuntu (gnome3, gnome3-staging).
Probably you also wanted to know whether there is an easy way to install it. The answer is no. Why?
- Because apparently it is not compatible with Trusty, too unstable on it and no one have yet it officially ported.
You can try to compile and install it by following the official Gnome compile guide.
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.