A HUD can replace a fail passive autopilot system in certain installations. With that a 737 and other planes, like CRJs can be CAT IIIa approved. Only ever tried the HUD thing in the simulator, as we had fail operational 737s anyway and therefore did not have a HUD as that would be just wasted money. But apparently it works quite well.
And yes, Airbus never really bothered with that to begin with. They do offer optional HUD installations, but seeing how few airlines opt for them i guess there is no real added value in them.