Some issues were reported on Steam last night, as both Slay the Spire 2 and Marathon launched at the same time. With one game being an extraction shooter, and another being a deck-building roguelike, it's difficult to see how they could've poached each other's audiences, but it seems that one has clearly won out over the other on the PC platform.
Via SteamDB, when we look at the numbers for Marathon's launch, they top out at 88,337 for the 24-hour peak. This is likely to go up this weekend, as more people get the chance to jump onto the game after a busy time at work. However, when we look at Slay the Spire 2's numbers, it's clear to see where Steam players are preferring to spend their time, as the peak player count over the 24 hours since launch is a whopping 288,314.
There are a number of factors as to why Slay the Spire 2 is monstering Marathon this way. It's a sequel, first of all, meaning it already has a pre-existing fanbase. Also, Slay the Spire 2 in Early Access is only playable on Steam, whereas Marathon can be bought on consoles too. Slay the Spire 2 is a cheaper game as well, although that's probably not a leading factor in people deciding which game to play.
Marathon probably shouldn't worry too much about the rise of a card-based deckbuilder, and should instead look at something like ARC Raiders for competition. Embark's extraction shooter continues to dominate Steam, having a 24-hour peak of 192,003 players, more than 100k over Marathon.
