In my limited experience many of the Regulations and FARs are based on what people thought was sensible back in the day when men wore leather and rules were few.
Could it be that 1.3 Vso was not originally "chosen" by anyone in particular, but was used in the days of biplanes as a reasonable speed at which to cross the fence? Any slower and you'd not have energy left to flare, any faster and you'd be too fast to achieve the desired three-point landing attitude in a reasonable distance.
O8