Because the intention of ICC profiles is to provide conversions from one device space to another, and 99.9% of the time, people want white on one device to be white on the other.
The way this is done is to either measure the devices with a D50 illuminant (print, transparency), or chromatically transform the device white to D50 (the Profile Connection Space white). This naturally gives you an overall transform (Device → PCS → Device) that is Relative Colorimetric intent.