Why might Shelley have capitalised the words "King of Kings," "Works," "Mighty," and "Wreck" in his poem "Ozymandias"?
You need to be aware that different text editions of this poem may or may not have the capitalisation that you refer to. However, having said this, it is always important to consider why some authors or poets choose to capitalise certain words and not others. Capitalisation is another form of giving special emphasis or meaning to particular words. You might like to think about what this tells us about these words. Note how the capitalisation in the words "King of Kings," "Works," and "Mighty" serves to heighten and emphasise the intense irony of the sign declaring the incredible kingdom of Ozymandias. This of course is underlined by the capitalisation of "Wreck," which refers to the actual state of the mighty kingdom of Ozymandias--now reduced to nothing more than sand.
Thus capitalisation is normally used by poets and authors as a form of emphasis, which can link particular words to the overall theme or message of the text. Here, the capitalisation is used to underline the irony of the sign that the traveller finds in the middle of the desert, making this poem an excellent meditation on the ephemeral nature of man.
All but the word "wreck" of the words you listed were engraved "on the pedestal" of the statue remains. The inscription obviously represented the spoken thoughts of the deceased man, who considered himself to have been supremely powerful and superior to all other men in the world at the time he lived.
As a reflection of the emphasis Ozymandias would have communicated regarding his position and the significance of his accomplishments in comparison with those of any who might "look on" the remains of his life's work, Shelley could have capitalized words that Ozymandias would have considered proper nouns - names of specific feats.
"Wreck" could have been capitalized as a point of irony - the mighty Ozymandias, reduced to "a colossal Wreck" around whom "nothing...remains" of his great accomplishments.