The thinking is that unique properly only describes what is unequaled or otherwise distinct from all others. Just as something cannot be more "only" than another, it cannot be more unique than another.

UNIQUE definition: existing as the only one or as the sole example; single; solitary in type or characteristics. See examples of unique used in a sentence.

if something is unique to a place or situation, it only exists in that place or situation:

very special or unusual. The preview offers a unique opportunity to see the show without the crowds. This house has many unique features, including a 45-foot-long outdoor swimming pool. He …

unique 1. used to mean 'the only one' If something is unique, it is the only thing of its kind. This is a unique opportunity. Humans are unique because they have the capacity to choose what they do.

You can use unique to describe things that you admire because they are very unusual and special.

Unique is normally taken to describe an absolute state, i.e. one that cannot be qualified. Thus something is either unique or not unique; it cannot be rather unique or very unique.

The platypus's nose is unique--no other animal has such a snout. If you're not sure whether something is, in fact, unique, try "special" or "unusual" to describe it.

Uniqueness is a state or condition wherein someone or something is unlike anything else in comparison, or is remarkable, or unusual. [1] When used in relation to humans, it is often in relation to a person's …

1. Being the only one of its kind: the unique existing example of Donne's handwriting. 2. Characteristic only of a particular category or entity: a weather pattern that is unique to coastal areas. 3. …