1.3 Vso is to take into consideration not only for the approach phase of the landing, but also for the go-around – just imagine flying at, say 1,1 Vso and then try to go-around – on most low-powered aircraft you will probably mush down to the runway, and this may also occur on some jet aircraft flying on the reverse side of the speed-power curve.
For what refers to historical development of the concept, I am pretty sure that the 1.3 figure follows from years of experience, analysis, certification flying testing, millions of flight hours , and – unfortunately, thousands of accidents.