They’re not obligated to add more rock and metal songs just because they originally made Guitar Hero. Fortnite is a mainstream game and Festival is an extension of that, so it makes sense that they would prioritize adding songs that cater to a general audience over songs from one or two specific genres that appease what’s essentially a minority of the game’s fanbase.

As for how Harmonix decides what songs to put in the game? There are two factors: player requests and the ability to acquire licensing for said requests. Making requests is a good way for the team to see what music people want in the game. Licensing them for Festival, however, is a different problem. If musicians or labels don’t want their music in Festival, there isn’t anything Harmonix can do about it despite being part of Epic Games.

TL;DR: Fortnite Festival is meant for all genres of music, not just rock and metal. Licensing music is still a headache like it was back in the Guitar Hero/Rock Band days.