Because scritta must agree with risposta.
In Italian, adjectives usually agree with the noun they describe in gender and number:
Compare:
For the same reason: adjective agreement.
Chiara comes from chiaro (clear), but it changes form to match risposta, which is feminine singular:
So:
Yes, it usually implies some kind of comparison, even if the second part is not stated.
So La risposta scritta è più chiara can mean:
Italian often leaves the comparison understood from context.
If you want to state it explicitly, you can add di:
In Italian, adjectives often come after the noun, especially when they are descriptive.
So:
That said, adjective placement in Italian can sometimes vary for style or emphasis, but in this sentence the normal order is:
And then after the verb:
So the structure is:
Here it functions as an adjective, even though it comes from a verb.
It is the past participle of scrivere, but in this sentence it describes the noun:
So grammatically, it behaves like an adjective because it agrees with risposta:
This is very common in Italian:
Yes, but that means something slightly different.
La risposta scritta è più chiara = The written answer is clearer
This treats written answer as one noun phrase.
La risposta è più chiara se è scritta = The answer is clearer if it is written
This adds a condition: it is clearer when/if it is written.
So your original sentence is shorter and more compact, while the second version spells the idea out more explicitly.