I’m sure there’s some reason behind this, but it feels arbitrary and weird without explanation. As if these genres specifically don’t deserve to exist…

Most game jam games are one of these two games. Simply pushing a large portion of the game devs out of their shell! Platformers and VNs are simply too common. Not that they're bad or don't deserve to exist.

(Not an organizer, but this is true for other jams that I've participated in too)

Hey, this is Kuba, the organizer - my colleague earlier made a lot of sense here. We generally love platformers and visual novels too, but those are the most popular genres in game jams, and without this rule, we often ended up with a majority of quite similar titles. That's why at Steelworks, we kindly ask you to choose any other genre for your game :)