In the time Fallout: New Vegas in set, Colorado, Arizona and I think parts of Utah are the Legion homeland. They want to make New Vegas their capital, not their entire homeland to the exclusion of all other regions. Particularly those where Legion rule and culture are well established. And they are indeed very safe and peaceful. Traders especially love operating in Legion territory because raiders are non-existent.
That's largely what makes the Legion such an interesting faction. Under normal, more civilised circumstances, they'd be seen as horribly barbaric, and rightly so. In New Vegas, on the other hand, they certainly have some very legitimate merits. The NCR at a glance operates in a far less repugnant manner, but that doesn't necessarily translate to a better life for the average person. Even in NCR home regions, raiders and other assorted bellends are still a major problem. Having a government that condemns slavery and torture is small consolation when it for one reason or another can't or simply won't stop the local outlaws from stealing your stuff, burning your home and raping/murdering your family. The Legion has no such problems in its own homelands. Even if you're a slave, you're still a hell of a lot safer than you'd be almost anywhere else, not to mention you're guaranteed food and shelter, which is more than can be said for the vast majority of wastelanders. It's really a hell of a lot less cut and dry than it seems at a glance.
Of course, all of this assumed that Legion home territory is indeed as safe and peaceful as we've been told. I really wish there was some DLC, if not parts of the main game, set in Colorado proper, just so we could see for ourselves what Legion rule would be like once the war is over.
In any case, I really like the factions in New Vegas. Not only are the NCR and Legion both shades of grey rather than 100% good and evil, But also the Brotherhood of Steel is not one of them. In every single fallout game, they've been be the major "good" faction of the game, and it would've been so easy and safe for Obsidian to go the same route with New Vegas. Instead though, they did what actually makes sense. The Brotherhood is simply too set in its ways of just scavenging and reverse engineering the fanciest guns and armour, and in the early days of the apocalypse, that was all you needed to be the best. Now that folks like the NCR, Legion and New Vegas itself have moved on from scavenging and began to rebuild modern states, the Brotherhood is simply no longer relevant.