I would imagine there aren't many disadvantages to keeping your walking-related bones solid and marrow-filled and your flight-related bones hollow. Plus, the flight-related bones which you have to waggle around all day at high speeds are next to your basal vertebrate lungs, which I would guess is easier to evolve air sacs into than the legs.

Thinking about all the chicken bones I've gambled with gnawing into over the years, I think maybe a lot of bird vertebrae have marrow, too. Unfortunately, that part of the chicken tastes kinda gross usually :/

I've also seen air sacs mentioned in vertebrae, in some context (help, paleontologists/ornithological anatomists?), so maybe those can have both? Is it neck vertebrae ('cervical'? It's been years since that 'Dinosaurs' class in undergrad...) that have the sacs? Am I thinking of sauropods, not avians now? Lotta cobwebs in this part of my brain...

Flight bones ('arms and hands') are fully hollow, spines have marrow and air sacs, walkin' bones ('pelvis, legs, and feet') are usually marrow-filled? Is that broadly true of bird skeletons?