I'd tend to say that it is b/c VCI and PRI measure aspects of intelligence that are more salient to giftedness whereas WMI and PSI measure processing skills that enable one to perform well in school. I've seen kids with high PSI and/or WMI and average VCI/PRI who are very high achievers in school, but who lack the kind of depth and abstractions that I associate with giftedness.

I've also seen kids with very high VCI/PRI and lower WMI and/or PSI who may not always appear to be the types of kids who get ided as gifted in schools (not convergent consistent high achievers), but who are clearly gifted.

I, too, think that having high WMI (and high PSI for that matter) are truly helpful in a school setting and probably in life in general, but I also think that they aren't the things that make one gifted.

Study Strategies for Accelerated Learners