It also resulted in romulans falling back to their own territory and allowing for federation control of planets like Kotaari which is a huge boon for its dilithium reserves alone. The romulans also thought the federation was developing WMDs after the Tomed incident essentially turned an entire sector of space into an inhospitable wasteland. They probably also didn’t want to start another war on the scale of the first romulan war, especially with the romulans having no hesitation when it comes to war crimes.