Here's another explanation or factor:
View attachment 113587
Should be self-explanatory, but that's the release schedule of all 5E books and box sets, categorized by basic type of product. We can quibble with details, but I did this quickly because I thought it was relevant: the rate at which different types of products are being published.
The main thing to note, that is relevant to the OP's sentiment, is that there hasn't been much by the way of new DM, Players and Monsters books - those would be the traditional "splats" - since Mordenkainen's came out, over 15 months ago in May, 2018 (Even though I categorized the two box sets as DM products, they aren't offering anything new so don't really count). The focus has been on, and will continue to be on through the end of the year, Adventures and Setting books.
Presumably this means that there won't be a new splat until the 1st quarter of 2020 at the earliest; given the fact that
Curse of Strahd is the only 1st quarter release in six years, it is more likely 2nd quarter - which means about two years between splat books.
So it may be that the OP either consciously or subconsciously is being influenced by this.
I'm not making a judgement either way--whether towards the OP or WotC--but I do think this is worthy of mention. I think it also implies that, given that WotC is very conscious of the frequency of types of products they publish, we'll almost certainly see a significant splat book come 2nd quarter of 2020.