In general? Because people often don't have a second player with them to help test while rating games lol. By definition, 100% of players are able to play single player (most people I know are one person), and only a fraction of players are able to play multiplayer.
So, with limited development time, it's best to focus on single player in a jam. But, adding in multiplayer as a bonus mode is definitely something I'd want to do down the line!
Thank you for the response! I was going for games published even outside a JAM. I remember Pocket Monsters#1 (GameBoy) to have multiplayer, albeit the battles appeared to be a bonus to the main single player game. The previous year, I wrote as answer to a question that I was verifying using javascript for GAME OFF 2021; however, the sample outputs that I observed appeared to not yet reach GAMES executed on the Super Family Computer. A few months before GAME OFF 2022, I learned of another ITCH.IO JAM, the GAMEBOY SHOWDOWN, where the outputs of using javascript caused me to verify it and use the know-how in our team’s entry. Perhaps, we shall soon find a JAM with multiplayer, e.g. make a FIGHTING GAME in one month.
Thanks! Here is the link to the GAMEBOY SHOWDOWN: https://itch.io/jam/game-boy-showdown
