Because Roddenberry didn't like war. He didn't like conflict. We were supposed to have evolved beyond the need for any base instincts of aggression, and the advancement of replicator tech basically made needing fighting over resources obsolete, hence the whole "no money in the future" nonsense, which has been contradicted a million times over on-screen. Never mind the fact that if other aggressor species did exist out there, they wouldn't necessarily embrace the whole notion of "peaceful coexistence" that the Federation (the Homo Sapiens-only club) espouses. Conflict is life, and makes for good storytelling. Roddenberry did his own franchise a great disservice by going all peace-love-and-flowers hippie horseshit for early TNG.
Things like the Cardassian War were retconned into the universe by the writers to tell new stories, but I agree they probably should have put that farther back in the past, prior to the E-D's launch. It didn't make sense that it would have been happening during the show's run without a single person talking about it. Then again, you'd never know that the Dominion War was going on during Star Trek: Insurrection if it wasn't placed in some throw-away dialogue early in the script. Any way you cut it, it's just simply bad writing.
Things like the Cardassian War were retconned into the universe by the writers to tell new stories, but I agree they probably should have put that farther back in the past, prior to the E-D's launch. It didn't make sense that it would have been happening during the show's run without a single person talking about it. Then again, you'd never know that the Dominion War was going on during Star Trek: Insurrection if it wasn't placed in some throw-away dialogue early in the script. Any way you cut it, it's just simply bad writing.