The fact that it is not a direct sequel is completely irrelevant to the fact that it isn't Fallout 4
I understand now - we've had people in the past ask assume this game was or should be Fallout 4 and I assumed this was in the same vein.
Ultimately New Vegas is not Fallout 4 because Bethesda decided as much. I realize that's rather arbitrary and risks turning the debate towards needless philosophy and abstraction but when you boil it down that really is the reason. If Bethesda had wanted New Vegas to be Fallout 4 then it would have been - they did not. I'd assume Bethesda would like to develop the core series from this point forward and if New Vegas was titled Fallout 4 people may assume they were passing the reigns to another developer.
I don't see anything in the official FAQ about this not being a "direct sequel" but it wouldn't surprise me if this has come up in interviews with Bethesda. In that case I'd guess they're using that explanation for two reasons. First, I think many people asking about that are wondering if it is a direct sequel. We get threads with some regularity asking whether people will be able to import their Fallout 3 characters to New Vegas (some who have even lept to the conclusion that this must be in), as well as other people who feel that this ought to be Fallout 4 for one reason or another. Second, its ultimately not very important who the game is named so I think the dev's try and keep the response short rather than going in to an in-depth explanation on why they chose to do this.
If it annoys you that people suggest it's because it isn't a direct sequel I'm not sure how to help you - however being overly hostile about the topic certainly isn't the way to go about it. This thread has gotten heated and I have to admit I don't see it improving even with a caution so it will remain locked - hopefully my explanation has helped.