The one disadvantage Nemrod didnt mentioned is the added required maintenance involved with the hoses (and they are perceived as delicate). In my opinion, this was one of the primary reasons the double hose lost out to the single hose popularity (the lack of a HP port for SPG was number two, IMHO).
In the 70s a Royal Aqua Master was regarded a better breathing regulator than most, if not all (single or double hose) regulators. The position sensitive (Items 1 and 2 in Nemrods list) was well understood but it wasnt an issue. If you look at the drawing from Bill Barada book Lets go Diving (look below) you can see that there are tradeoffs in regulator positioning.
If you restrict yourself to swimming exclusively in a horizontal position, a single hose regulator has the advantage, but in a 3 dimensional world (like an underwater reef) I never restrict myself to only being horizontal, therefore the single hose tends to loose that advantage.
In the 60s and 70s there were several DH regulators that were hard breathers (Dacor, some Healthways, and several others), but even they were probably better than some of the single hose of that time period (USD Aquamatic, Healthways Airstream, etc.).
Back to my point on hose maintenance:
The hoses are perceived as delicate (in Sea Hunt they often got cut accidentally inside a wreck), but they really arent. I have never seen a good hose getting cut (not to be confused with a deteriorated hose). The navy has used and continues to use DH regulators and some firefighters SCBA use very similar type of hoses. The problem is when the inside of the hoses get wet you have to rinse them and dry them or you will get bacteria growth.
For most people it is enough of a hassle having to rinse or soak the outside of all the gear after a dive. Having to take apart the hose assembly to rinse the inside and dry it is an additional hassle. I dive almost every weekend, so you can see that I replaced the screws on the clamps at the regulator end with thumb screws. Other modern DH divers use quick release plastic clams and I know of one diver that buys zip ties in bulk to use as hose clamps and cuts them after every dive trip.
I am not saying that the increase maintenance by itself killed the double hose in the 70s. But, I think that was the first step, closely followed by manufacturers lack of improvements such as adding HP, LP ports and a more robust exhaust valve (something more durable than the original duckbill), etc.
Here is the diagram I mentioned:
From "Let's Go Diving", an Illustrated Diving Manual by Bill Barada, Published by U.S. Divers Co., 3323 West Warner Ave., Santa Ana, CA. Catalog No. 7804-00, Copyright 1962, 4th Printing November 1965, page 29.