NCR can't send troops in from the North because of the Divide and Big Empty. Which means the Legion can't invade that way. NCR has to send troops around and through the Rocky Montains and the podwer gangers stopped work on the rail line.
Still it is the only way for the Legion to invade and CONQUER NCR, which is their Goal. The Mojave is not the main target of the Legion. Caesar will not stop once he takes the Mojave. He wants all of NCR. This means Mojave will be the only way to send troops into the NCR. So Legion armies will flood the Mojave.
Simple enough? Do I need to get out the hand puppets and a whiteboard?
Saying things like 'dumb down' and hand puppets doesn't help you be more right you know?
Check a map, the area surrounding las vegas is in no way the quickest or easiest way for the legion to get to ncr lands, they go to the mojave in the game because of the ncr being there and because caesar wants the strip..
Both are voided in this debate.