Because it takes the worst aspects of Rome and Sparta mixed with Genghis Khan's Mongol empire. Caesar doesn't have time to set up a strong successor before he kicks the bucket, and it will quite obviously collapse into feuding warlord states as soon as he dies. Plus willingly charging into machine guns with endless meat fodder will clearly lose the war of attrition over time. You really think the NCR has its full power deployed in Nevada? They have like one army base and a few outposts. They're replying on the depleted rangers to slow them down and aren't taking the Legion as a serious threat to the NCRs existence.