I think it has to do with pricing and MOQ. If a set isn't very popular it can be hard to get to MOQ for just the base set, there won't be any extension available then/ or very expensive. But for a very popular set like 1976 it's possible since it will always fund + massdrop helps funding.
You're implying that if a set is unpopular, then it will be hard enough hitting MOQ for the main set. Therefore offering alternate sets is kind of pointless, with which I agree.
However, say you were offering a base set and an optional ortholinear modifier kit; I can see why ortho users would be deterred from getting the base set in addition to the modifier kit: it's more expensive and wasteful than getting only what you need. So wouldn't a standalone set simply boost the ortho option's popularity?
From there, we could argue that offering an alphas kit + modifier kits is exactly "getting what you need". Yet, I could see that (in the case of single colour sets) everyone is getting alphas anyway, so why not include them with each modifier kit? I will have to confirm with the manufacturer, but don't larger sets mean less $ per key?
