The first question is difficult to answer with certainty. You can try to say the problem is “quality,” and that is certainly at least partially true in some cases. However, it is not necessarily true in the case of two of the biggest games mentioned above: Marvel’s Midnight Suns and Guardians of the Galaxy. Perhaps those games weren’t for everyone, but they were both incredibly well-reviewed titles from some pretty big-name studios.
Yet, the studio involved may be part of the problem in the case of Guardians of the Galaxy. No, I’m not talking about the exceptional job that developer Eidos-Montréal did; I’m talking about the stigma surrounding Square Enix’s Marvel projects following the failures of Avengers. Despite the many differences between those games, their similarities proved to be a little too close for comfort for the recently burnt. Both were “from Square Enix” and both gave off “kind of MCU, but not really” vibes. It’s also not like the Guardians of the Galaxy marketing team could simply state that one game wasn’t actually being made by the developers of that other Square Enix Marvel game you hated.
For that matter, it probably didn’t help that Guardians represented a departure for Eidos-Montréal who hadn’t released a major new game in quite some time and were best known for their rebooted Deus Ex titles. You could say something similar about Midnight Suns. It was, after all, a game made by the XCOM studio that looks kind of like XCOM but was actually a card-based strategy game (though not a CCG). The game was largely exceptional, but so much time was wasted trying to communicate what it is and why people should be interested in it.
Communication has been something of an issue with Marvel games, in general. The MCU has trained modern Marvel audiences to expect a fairly uniform experience. Similar styles, familiar faces, and continuing storylines. The decision to divorce recent Marvel games from the MCU is seemingly ideal from a creative standpoint (more on that in a bit), but don’t underestimate the confusion it has caused.
A Guardians of the Galaxy game released years after one Guardians of the Galaxy movie and years before another? One Marvel game that’s a fairly linear action-adventure title, another that’s a card-based strategy game, and another that’s a Destiny-like live service game (none of which were handled by studios with notable experience in those genres)? The presumed target audiences for these games don’t always know what to make of them at a glance, and their eventual quality isn’t necessarily enough to save them from rapid obscurity as a result of that confusion.
Right now, Marvel games vary wildly in terms of their quality, pedigree, genre, and releases in relation to notable MCU projects they may share some elements with at a glance. Some of those factors shouldn’t matter as much as they do, but if we’re just talking about sales performance, then they clearly do matter to at least some degree.