There are generally 2 reasons to avoid garlic and onions - 1) they are considered 'stimulating' (eg: Krishna) and/or more likely: 2) onions and garlic are bulbs

There are generally 2 reasons to avoid garlic and onions - 1) they are considered 'stimulating' (eg: Krishna) and/or more likely: 2) onions and garlic are bulbs - if you uproot them you effectively kill the plant whereas other vegetables you harvest their fruits/leaves/etc and the plant lives on.