TNG was basically Star Trek with new characters and some tweaks ergo it was essentially the same show and with the same format. That format was established by Roddenberry in 1966. Roddenberry apparently did some writing on the show bible first, and was involved in many of the creative decisions that shaped the show. As such the additions and suggestions by Gerrold and Fontana did not constitute enough of a difference that the WGA was going to give anyone but GR a coveted "Created by" credit.
But there are other cases where a spinoff with the same format but different characters
does give co-creator credit to its developers, if not full creator credit.
CSI was created by Anthony E. Zuiker, but
CSI: Miami and the other spinoffs credit Zuiker, Ann Donahue, and Carol Mendelsohn as the creators.
NCIS was created by Donald P. Bellisario and Don McGill (itself a spinoff of Bellisario's
JAG, but with a different format), yet its spinoffs are credited solely to Shane Brennan for
NCIS: Los Angeles and Gary Glasberg for
NCIS: New Orleans -- in both cases, the writers of the shows' pilot scripts, with the creators of the parent show not being credited at all. Going back decades,
Mayberry R.F.D. was a direct continuation of
The Andy Griffith Show without its title character, but TAGS was "Created by Sheldon Leonard" while MRFD was "Created by Bob Ross."
On the other hand, all the
Law & Order shows are credited solely to Dick Wolf as creator, even though he only wrote the stories for the pilots of the
Criminal Intent and
LA spinoffs. So there is precedent for how it was done on TNG, granted. But there's also precedent for sharing creator credit.