Yes I understand why this merchandise exists and that there are lots of people interested in it, I too own some. My biggest gripe with the merchandise is that it's barely related to in-game items but instead thing like candles and underwear (although that has changed a bit since Bethesda launched their new store) but most of all that there isn't any merch for games like Fallout 1/2 and New Vegas, even 3 barely got any. It's like they're pretending those games don't even exist.
As for the games themselves, seeing how Fallout 4 turned out and now 76, I just don't have any hope for 5 or other future games unless they give some outside studio another try, which'll probably never happen again..
IIRC there was merch for FNV. I recall NCR T-shirts. Might be something else, can't remember.
Fallout 3 had collectible bobbleheads, as well as a lunch box.
As far as F1/F2 goes, it's more or less given why there is no merch - they are old games, and while iconic among the older RPG crowd, are hardly popular among modern gamers which are the target audience of F4 and of this kind of merch. As a side note, F2 had little merch back in the day. Very little though, it wasn't as big as it is now.
As to why FNV and F3 are severely lacking in merch compared to F4 - it's because at the time those games were released Bethesda wasn't dabbling with merch a lot. They made a bigger move with Skyrim and later on with F4 in that direction. Prior to that, their collectible items were not a huge thing.
It should be noted that they retroactively capitalized on more iconic aspects of FO3 which are present in FO4. Portable nuke, T-45 Power Armor figure, Pip-Boy 3000 etc. Given FO3 introduced these, you have FO3 merch arguably.
It is true there is little merch based on stuff FNV introduced. We all could guess why, but at the end of the day, it would just be our suspicions, but no hard evidence, regardless of how well that suspicion is rooted.
At the end of the day, I don't think Bethesda is pretending older titles don't exist. They are just capitalizing on the success of the currently the most popular thing, and that is FO4 and upcoming FO76. Good business, nothing more.