Maybe a trivial question, but why long hose and bungee necklace setup are better than the default configuration of short hose and octopus in D-ring,
As several have mentioned, the primary advantages are a) donation of a 'working' regulator in an OOA situation, and b) ready accessibility of the back-up for the donor. I personally find the configuration to also be c) safer for the diver in the event that their second stage is knocked out of their mouth by another diver, i.e. their alternate is right beneath their chin, and there is no frantic searching for it somewhere in the (Bermuda) triangle, d) more streamlined, and e) easier to swim with a buddy, either as donor or receiver, when a long hose is used for the donate reg. But, that may just be me.
One functional alternative to the 'long hose' part (the hose length and cost seems to be, for some, a sticking point in considering such a configuration), which still provides 4/5 benefits, is a configuration that a number of university scuba programs seem to be adopting - the diver breathes from the alternate / octo on the longer (40") hose as their principal second stage, and puts what would have been their primary second stage (on the shorter hose) on a bungee, positioned below their chin. I really like that configuration (and even proposed it as a change in our shop OW training standards - we decided not to adopt it across the board because many instructors still dive what would be considered a more traditional rig). In such a set-up, the alternate is still 'clearly marked' - the same yellow hose / yellow purge cover / yellow second stage housing / whatever is used - and the alternate still resides in the 'triangle' that is frequently described and taught in OW. In fact, the second stage is very visible to an OOA diver, at the very top of the triangle (in the diver's mouth), not obscured somewhere toward the lower corner.
FlyingSquid:
and since better why is it not embraced?
I agree that the timing of introduction of the set up has been a factor, as is simple lack of awareness of the configuration (among many divers and instructors). But, I also see it as a matter of inertia and complacency. The likelihood of an OOA situation requiring donation of an alternate is really quite low. Many divers intentionally use (and get away with using) lower performance (aka less expensive / cheap) second stages as their alternates because they don't think they will ever actually need them (and usually don't). A number of these divers also, for whatever reason, don't seem to notice that the alternate frequently a) extends out to the side or below them, even while the second stage hose is clipped to their BCD, or b) comes out of the clip / mouthpiece holder / whatever they are using altogether, and dangles below them, gently bouncing along the reef or the silty bottom. Since it is hard to document real problems with OOA air-sharing with the traditional configuration - because the instances are rare to begin with - there is no impetus to overcome either the inertia of having started that way, or the complacency of continuing to do it that way. This is the 'If it ain't broke, don't fix it' approach. Nothing is necessarily wrong with it. Personally, as I have aged I have come to favor the 'I ain't gonna wait for it to break.' approach. But, again, that is just me.