It makes pretty good sense. BfA was the ideal for Shadowlands because it introduces the Horde and Alliance and Sylvanas (you have to do the Lordaeron intro stuff) and is also the most up to date question content besides Shadowlands, and is self-explanatory, not particularly requiring earlier knowledge (you don't need to know why the Alliance and Horde are currently at war, just that they are).
Dragonflight is unrelated to the previous few expansions. It's just about Aspects and an exploration party going out to explore, because right now happens to be the time where the islands are waking up. There's no ongoing story you have to at least somewhat explain, Dragons are looking for Dragonhome, easy. This means Shadowlands doesn't have to be the default experience.
So then they went "what should be the intro". Shadowlands didn't make sense, because it's a very awkward expansion for someone who knows nothing about Warcraft, you get off the island and suddenly you're in the land of the dead, without knowing what that is, helping people you don't know and hearing about characters (Sylvanas, Lady Proudmoore, the Night Warrior, etc.) you've never heard of. Shadowlands requires at least a bit of intro, which is why BfA was used before it.
So then your next logical step is to just keep using BfA: it's very modern and tight questing. It's a pretty solid entry for new players, you show up in stormwind and get recruited to go treaty and help the war effort. You get taught about the places (Zandalar, KT) you go to, so you don't need prior information because NPCs talk about the history and stuff.
It's a very good expansion for brand new people and is low concept, where Shadowlands expects referential knowledge and is the end of several storylines.