[quote=@Gisk]
I'm sure they wouldn't be the first to make that journey.  In fact, Suzy's character is stated to have done so.  I don't think you need to justify it at all.  We can assume the trip isn't easy, but it's not impossible.  Especially with groups like the Brotherhood and NCR trying to civilize the wasteland.  There's a good chance there's some sort of road or system to get people across.  
[/quote]

But the NCR canonically aren't anywhere on the East Coast, and the first (and only) time the Brotherhood tried to make the journey east (in GIANT ARMORED ZEPPELINS), they were very nearly wiped out thanks to the constant tornadoes and splintered into multiple chapters.