To be fair they wanted to use John Colicos as Kor in three more episodes over the next two and a half years TOS ran; but he was busy in theater and when they needed him his schedule never allowed it. He did say in an interview he would have loved to have the opportunity to reprise the role during the original series; and it was why he was happy to have the opportunity when DS9 producers offered it to him.
They also wanted to bring back Tige Andrews as Kras for "The Trouble with Tribbles," which would've been interesting, given Kras's onscreen death in "Friday's Child".