I had installed Fortnite to play Festival when that launched to get my Rock Band fix and yeah, Rocket Racing was great for a free arcade-style racer.
Both Festival and Rocket Racing seem like they've been on life support for years, with just a couple new instruments / the paid-for musician model and new tires every season. I imagine Racing was even uglier to monetize, as you can only sell car models, which are probably encumbered with all sorts of exclusivity deals between Forza, Gran Turismo, etc.
(I can't speak to any other game modes as I've only hopped into the Simpsonified island to continually think "Man, I'd love to appreciate all the work they put into this, if i wasn't getting killed by 12 year olds instantaneously")