I'm not exactly sure but I think it's because of the rarity of this condition. To be able to determine the prevalence or incidence of PVS, a study probably needs a sample size in the hundreds of thousands, if not millions. Moreover, the sample size has to be selected in an unbiased manner.

Existing national medical databases probably did not record PVS symptoms, making it very difficult to track PVS symptoms as scientists did for known medical conditions with ICD-10 codes (e.g., stroke, heart disease, etc.).