My original post was a little longer with more steps in the funnel, but I decided to simplify things. I haven't bought any WotC products since Tasha's, so I'm not familiar with the new Spelljammer release, but I remember reading that people were disappointed that it was missing stuff that they believed should have been in it. I can see people not only wanting the missing stuff but wanting official versions of it. Officialness has a really strong pull, and there are absolutely people who won't even look at third-party products because they want things that are official, or canon, or whatever.
What I was getting at, thought, is that although there are a lot of 5e D&D players, the funnel narrows pretty dramatically when you're talking about anything beyond the Player's Handbook, and then can narrow even more depending on other factors. According to the sales figures for D&D books that leaked last year, we know that Spelljammer had only about 5% of the sales of the PHB. Some of those sales were to people who just buy everything in the D&D line and don't necessarily plan to use it. Some were to people who wanted access to the new races, classes, spells, magic items, and monsters. Some were to people with an interest in running or playing Spelljammer, but who haven't gotten around to it. Etc.