Screenwriter Gene Roddenberry revealed that the only reason he returned was anger!

Gene Roddenberry was an American screenwriter and producer known for his work on the Star Trek franchise during its earlier stages. Although Roddenberry has several other projects to his name, his fame and acclaim grew with the franchise.

When Star Trek wanted a Phase II, the producers didn't want to work with an unknown person. Well, that's when they approached Roddenberry to write the script, but he declined the opportunity and he had a very valid reason to do so.

Star Trek: The Original Series ended in 1969. Writer Gene Roddenberry intended to continue the franchise and take it in a better direction with Phase II of the franchise.