The Isosceles Stance, is based on the "isosceles triangle." Anyone steeped in mathematics, engineering or drafting, would have encountered the use of an "isosceles triangle".
So what is it, it is a triangle where two of the sides are of equal length, forming equal angles, similar to this image. The two longer sides come together with the shorter base to form a strong triangle, that can be easily pivoted.
This same triangle is created by using your body and outstretched arms hands gripping the pistol. This makes the Isosceles stance a naturally defensive stance that provides excellent coverage in most directions by simply rotating the upper body, to pivot and pan an area. If you were to look down upon a shooter with an Isosceles stance it would appear to be a triangle as shown below.