I would guess that chest surgery is typically just a quicker and simpler operation than vaginoplasty operations which require surgeons who specialize in that particular field, and there are probably less of those than surgeons who do mastectomies and top surgeries. I think it's just a rarer procedure done in general outside of private channels, than chest surgeries of various kinds so there will be less dedicated surgeons available. Maybe that is why wait time are shorter for those operations. I'd expect phalloplasty to be more in line with the vaginoplasty waits though.