I'm not particularly surprised.
I bought the game, played it, loved it (in parts), but I have to say a lot surrounding it was a streak of spectacularly bad decisions.
I had my reservations about the card thing, but turned out to be one of the easiest things to overlook, because the combat was incredibly fun and engaging at its core.
But then you had:
- a particularly flavor of Marvel narrative which is a niche inside a niche and never had too many fans to begin with. If this was a "standard X-Men" thing I could have seen it appealing to a lot more fans.
- a very high entry price with a hefty 50 bucks "Season pass" too, which in itself is enough to put off a lot of potential buyers.
- an "inconclusive dating sim" (since "no fucking with licensed characters") that no one really asked for.
- the attempt to monetize every little extra that compared unfavorably to XCOM 2 and its popular mod support.
- fairly terrible writing across the board.
I liked Midnight Suns just fine, but I don't think it comes even remotely close to match the quality of XCOM2- War of the Chosen.
And a lot of people must have had the same impression, because to put things in context XCOM 2 peaked at 133K concurrent users on Steam, while this one barely reached 15K.
The game flopped because it was a niche genre that thought it could be saved by Marvel licensing. So, someone greenlit paying Disney a grip instead of going the D&D route or using original characters which would have made the game less of a financial risk.
Being a "niche genre" is not an excuse in itself.
It doesn't explain why their previous, non-based-on-a-licensed-IP title sold 10 times as much, for a start.