Not really a surprise, the very quick and pretty big discount was only the last hint, that this didn't do too well. Marvel fans expected some action and not "card game strategy", it was priced very highly, it turned parts of the XCOM audience off (no xcom 3, card based, etc.). It had and has technical issues, the pacing is weird to put it nicely and it partially doesn't really look good for what it is. Don't get me wrong, I bought it during a sale and finished it, but I still think that XCOM 1+2 are far superior and some parts of Midnight Suns aren't just good. Also the fact that you have a season pass with full price announced directly didn't help I guess. During sale (GMG) the price was fine for what I got, aside frome the mentioned issues, but I would have still preferred XCOM 3 over this and we might not end up getting it at all, was Midnight Suns didn't do well (enough).
I don't think the time of the release was the biggest issue, a lot of people had doubts (to put it midly) in advance, I was basically only curious as it is from the devs of XCOM 1+2, but the card game and some other things put me off, until it was on sale and I won't buy the season pass until everything in it was released and that one is on sale as well.