Leaving aside the quality of Rock Band vs. Fortnite, there are some other key differences:
Playing with your friends in the same room. With Fortnite I think that’s possible on consoles, but it isn’t how people tend to play. If you’re not playing games with friends in the same room (at least sometimes) you’re missing out.
A variety of games. Almost nobody exclusively played Guitar Hero, or Rock Band. They were just one of many games people played. For some reason, kids these days seem to be hyper-focused on one game. First it was Minecraft, next it was Fortnite. My nephew switched to League of Legends next, and again, it’s all he plays. I can understand getting hyper-focused an MMO, because they try to pile in all kinds of content: quests, raids, dungeons, professions, seasonal events, exploration, PvP, PvE, etc. But, Fortnite and LoL lack a lot of those features.
Leaving aside the quality of Rock Band vs. Fortnite, there are some other key differences: