Why is there a lack of initial high vowels in Japonic high register?

While looking at Igarashi (2019)'s JR-COGNATES list UniCog, I noticed that most of the disyllabic high register nouns go back to initial pJ vowel *e, *o, *a, and *ə, but rarely the high vowels *i and *u. This could provide some internal evidence for Proto-Japonic *e and *o in some words otherwise unrecoverable by the comparative method. This is because there is high register nouns such as pR *medu 'water' (< pJ *meNtu 2.1 'id.'), others.