They may have sold well, but not well enough, since the relaunch of island of blood in early AoS was in fact a reboxing of unsold models printed far earlier.
Futhermore, starter set are not about pleasing an existing fan base: they are all about recruitment. That's why island of blood in 8th edition contributed to the doom of the old world. It did sold quite nicely among the existing player base, but failed to expand it significantly. Notably because none of the model inside was easy really they easy to paint or to sold to people anaware of the game or setting.
That's why Stormcasts doesn't need to even be the most popular faction to justify their presence in a series of starter sets. They just need to be beginner friendly, which they are. Because they need to appeal to young players, to not rebuke their parents (who got the money) and to be recognisable poster boy. And they need to be easy to paint to keeps the new players long enough, to encourage them to keep painting, playing, ans buying.
With this in mind, pitting religious zealots with a baroque iconography and numerous and detailed models against shadowy demons with fiddly bits as a starter set may please us, but will not help people from hobby center to recruit new players. Ence the fact that I, for one, doesn't not belive in a non Stormcasts starter set, and even more, doesn't believe in a starter set with a ultra detailed adversarie in front of them, but more with something rather simple to apprehend, like Nighthaunts, who can look nice with juste a wash over a base coat.
But i don't want to be all doom and gloom : we're still gonna have our humans and shadowy demons, and probably with a nice army set 😀