Possibly because of the first half-baked three-book release, Spelljammer. Which I'm still salty about.I'm not remembering why I avoided it.
@paradisebunny it won't be the core of your content, but since someone mentioned Kobold Press' upcoming planes release, I figured I'd mention that Monte Cook did a planes book a while back, Path of the Planebreaker, and EN's Level Up A5E has the Planestrider's Journal. I've found both of them to be very useful in running a Planescape game.. as for core Sigil/Planescape content, I still rely on the old 2e content for the most part.