Because ‘coin’, as a verb in this context, is only applied to the invention or creation of a new word or phrase and requires as an object a noun such as word, phrase, term, expression, but does not work with the definition of that expression alone.
The first example given in the Oxford Dictionary on line is in the active voice:
‘he coined the term ‘desktop publishing’’
although it also provides examples in the passive:
‘A new word was coined to describe such individuals: ‘cybersquatters.’’
And one can use the word adjectivally (my example):
‘‘cybersquatters’, a word coined to describe such individuals’
but, again, in relation to a word or phrase.
The incorrect adjectival usage in the example in the question would imply that you could say:
‘This phenomenon was coined a ‘computer revolution’’
Although grammatical, this doesn’t make sense. It is a phrase that was coined, and that phrase happened to be ‘computer revolution’, but ‘computer revolution’ alone refers to a phenomenon the occurred, irrespective of its name. It was not coined, it happened. So one must say something like:
‘The phrase ‘computer revolution’ was coined to describe this phenomenon’
In contrast, term does not mean to invent a name or phrase, but rather to use a name or phrase to describe a phenomenon, and takes as its object the phrase literal. The Oxford entry is:
Give a descriptive name to; call by a specified term.
‘he has been termed the father of modern theology’
The person that used the phrase did not necessarily invent it, and the sentence refers to the phrase itself, not to the action of inventing it.