Except for a while it wasn't part of the official live-action continuity.
Who did originally try to dictate that anyway? Was it Roddenberry?
By certain accounts, it was really Roddenberry's lawyer, and this directive was only put into place when TNG was being produced.
TAS was not a "reboot," in the sense that it did not scrap previous continuity and start over. TAS continued onward from TOS, even having episodes that were direct sequels to TOS episodes.
Kor