Exactly, the chasm was created by the Divide event which is why getting supplies and reinforcements to New Vegas is so tricky in 2281; it was just a city and you're suggesting that the non-collapsed parts are giving the NCR trouble. The NCR control the formerly largest populated - and one of the largest - cities in the United States - Los Angeles. Why would Hopeville - the city - give them so much trouble when it didn't stop an entirely independant town from eing founded there before the event.
EDIT: And I meant in the approximate area of that reserve, clearly if the lab was there it wouldn't be a nature reserve.
Oh no, I wasnt saying the distance initself. It's like.....Say you're marching across D.C. the city. You aren't walking a long ways, you're marching through broken buildings, radiation hotspots, monsters, raiders, et cetera.
In Hopeville's case, you have varying degrees of trouble. Unstable pass points from one point to the next, tunnelers, the dangerous winds and earthquakes, and the radiation that is rampant in the air. As what turned the soldiers into Marked Men wasn't just sand and wind and ash, it was also constant radiation. the Divide is more or less a literal 'hell on earth'.
Edit: Oh wait, we're talking about two seperate things here. I was figuring you were meaning just hopeville. Derp.
Apparently, from what I recall, which is vague, Big Mountain is North I believe of the Divide, which would make sense as it forces NCR to march the long way around both Hopeville and the Divide.