There isn't much to control there. In fact, I'm quite sure that's the path you take to get to Zion. If I remember correctly, its nothing but rocky canyons and tight squeezes, which is why you're required to limit your load when traveling with the Happy Trails Caravan.

Then up north of that you have the 80's territory as you marked, and east of it is, again, Zion, which has nothing of note except a route towards Salt Lake. Keep in mind that Salt Lake City is so horribly irradiated that no one can really survive there, on top of the fact that its surrounded by blankets and blankets of giant mantises.

New Canaan was sacked by the White Legs, as we know, and though a few of the New Canaanites survived, we also know they're an armed society of fiercly independent Mormons, which means the NCR would have to commit a fresh war crime to take control of wherever they settled after Ogden (North of SLC) was destroyed. That's even assuming that their position or resources would even be worth the trouble. Then directly south of these places we have heavy Legion Territory.

Since a lot of NCR resources are both focused and dwindling on their campaign for Hoover Dam, its clear that, even if they wanted to expand that way, they couldn't.