GNU contains a lot of non-GNU-written software because, to create a fully free OS, the only portions that needed to be written were the portions that did not have free replacements.
All the “Linux” distributions are actually versions of the GNU system with Linux as the kernel. The purpose of the term “GNU/Linux” is to communicate this point. To develop one new distribution and call that alone “GNU/Linux” would obscure the point we want to make.
As for developing a distribution of GNU/Linux, we already did this once, when we funded the early development of Debian GNU/Linux. To do it again now does not seem useful; it would be a lot of work, and unless the new distribution had substantial practical advantages over other distributions, it would serve no purpose.
Instead we help the developers of 100% free GNU/Linux distributions, such as gNewSense and Ututo.
https://www.gnu.org/gnu/thegnuproject.html
GNU contains a lot of non-GNU-written software because, to create a fully free OS, the only portions that needed to be written were the portions that did not have free replacements.