Why vowel signs of Indian languages are not considered as combining characters in Unicode?

In Unicode database for Indian languages like Hindi, Kannada, Telugu etc, why vowel signs have combining class property equal to ‘Not Ordered (0)’? What is the technical reason that they are not categorized into combining characters even though they modify their base glyph? What is the criteria for a combining character? Surprisingly though, ‘nukta (7)‘ and ‘virama (9)‘ signs are categorized into separate combining classes.