DevonDiver
N/A
In addition to rongoodman's comment, it's also because tucking a long hose in your jacket cumberbun creates a weird bow (stealime as it may be) but a weird bow non-the-less across your front side. The shoulder straps prevent you from cleanly keeping your excess hose tucked on your side, and even worse, the lack of proper fit with stock BC's means your long hose may often come untucked and flap around..
It's worth remembering the 'non-Hogarthian' alternative. Slip a few loops of bungee around your cylinder and 'stow' the hose there. Retain sufficient length to run under your arm, around your neck and into your mouth. This provides for an immediate donation of the air source, followed by a quick deployment of the remaining hose length. The drawback is that it is much harder to re-stow your long hose (especially without buddy help or removing your rig completely). This prevents S-drills on descent - a very good protocol that, along with descent/bubble checks is rarely educated to recreational divers. S-drills, bubble and descent checks are definitely something a 'tech flavored' recreational diver should investigate... prepping for tech in the long term should be more than just equipment familiarity..
BSAC still teach the bungeed long-hose as their preferred/authorized method of long hose stowage: “Hogarthian rigging” and “Primary take” when teaching “out of gas response” on BSAC courses
Personally, I don't like the 'BSAC method' - but then, I don't dive in a jacket BCD either..