Cars of course have thousands of moving parts while a messenger bag has...maybe 5? It's the same basic form factor. Fundamentally, one strap will be more uncomfortable than two, there's simply no way around it. It makes sense if you have to swing your bag around all the time, like a mail carrier or a ... bike messenger... but for the rest of us it makes no sense at all.

Assymetry is neat-looking though, that I will say. Thus, it's all about fashion.