Look, could the NCR send a small expedition to the East Coast? Of course they could. Could the Legion? Yes, to that also.
Could either send an expedition to the East Coast large enough to be a significant presence? They probably could, but they have every reason not to and no reason to do so, at least not while they are trying to consolidate power on the West Coast.
Well, the Brotherhood did it, why wouldn't the NCR or Legion do it? The Brotherhood of Steel has its back to the wall. They are losing, and if they lose, they lose everything. It may take years, even a few generations, but if they don't change the tide, they will wind up as a footnote in some dusty unread history book. And they know this. Sending an expediton East (actually two, but that is another story) makes a lot of sense. First, it ensures the Brotherhoods survival even if the West Coast falls. Second, there is a good chance that the expedition can get some weapons and other technology that will allow them to stave off defeat in the West. I will be the West Coast Elder's had a four hour long orgism when told about Liberty Prime. Obviously, things didn't work out as planned. It is also a sure bet that the West Coast Brotherhood wasn't paying attention when the canon ending of Fallout was revealed, because if they were, they would know all these expeditions were unneeded. They are going to survive (just not as they are).
Both the Legion and the NCR believe that if they just hold on long enough, the other side will fold. They both know they are in a weak position, but if they hold firm they can win. Sending a large force East will only weaken them, so that just isn't going happen. The risk is much too great for an unknown reward. There are not going to be any big expeditions for quite some time.
That leaves small expeditions. It is entirely possible for a small group of rangers or frumentarii to make it to the East Coast, but what would be the point? I am not asking for a Legion or NCR justification but a BGS justification. The only reason for Bethesda Game Studios to equip and man an expedtion to the East Coast is to tell us what the canon ending of Fallout NV is. This would pretty much straightjacket any future game written to take place on the West Coast. Fallout: Seattle (or whatever) would not be able to develop the history of the West Coast because it would have been done before development had started. It would actually make such a game LESS fun, because the whole "discover of what went on before" part would be missing.
Let Fallout 4 give us canon information on Fallout 3. Let Fallout: Seattle (or whatever) give us canon information on Fallout: NV.