Adjective ending "-ний"

Why are these soft adjectives, but "-ий" are hard? For example, синий, синяя, синее; not синий, синая, синое.

I'm guessing that this has something to do with the letter 'н', but I'm a little confused.

-ий itself is a 'soft' ending.

-ый is the hard adjectival ending because ы is a 'hard vowel'
-ий is the soft adjectival ending because и is a 'soft vowel'

He means why does "-ний" decline into "-яя", "-ее", "-ие" etc. instead of "-ая", "ое", "ые" etc.

...to which I have no answer. I don't know. It may be worth noting that in most other adjectives ending in "-ий", the stem ends with a "к", "ш" or "щ", consonants for which the spelling rules require that they cannot be followed by the letter "ы".

(But I really don't know if this has anything to do with it...)

Thanks :)