It's just the luck (or unluck?) of the USPS I think? I've shipped eggs to Florida that got 86% hatch rate.
unluck, most definitely! my postal driver is actually incredibly careful with the box (i've given him some eggs from the laying flock, so he has a vested interest), but it makes me wonder whether one of the transfer stations the eggs must move through to get to me is particularly rough, since i've had such consistently bad luck with shipped ones...