I think D didn't gain the momentum because of GC. O/B system is interesting but makes the syntax ugly (at least in Rust's case).
Had D gone with a (deterministic) ref counting like Swift and Vala, it would've been much more popular I think. Such memory management keeps the syntax cute :) while not sacrificing the determinism.
We looked hard at an automatic ref counting system. We could not find a way to make it 100% memory safe while allowing pointer access for efficiency. An O/B system makes this workable.
