Why does P.B. Shelley claim that poets are "the unacknowledged legislators of the World" in the 'Defence of Poetry'?

In his Defence of Poetry P.B. Shelley claims that poetry 'is connate with the origin of man' and man 'is an instrument over which a series of external and internal impressions are driven'. Shelly demonstrates the crucial role that language plays in the intepretation of impressions. He believes that the metaphorical power of language marks 'the before unapprehended relations of things' and then perpetuate their apprehension, until language transforms these relations into words, which represents them through images 'for portion and classes of thoughts'. Shelley's account of poetic language seeks to find an order to the chaos, which, possibly, Shelley sees in human society: a chaos that only poets can fathom. Therefore, poets' enhanced poetic language can re-institute an order to human society. For example, Shelley maintains that poets can institute laws and create new materials for knowledge, determining the role of poets as legislators. In calling poets 'unacknowledged', Shelley challenges the critic Thomas Love Peacock, who in the Four Ages of Poetry believed that the progression of society caused the deterioration of poetry. Shelley utilizes the Defence of Poetry to suggest otherwise and to condemn, covertly, a society that underestimates the importance of poets and their contribution to the progress of society. Therefore, poets are 'unacknowledged legislators', because their role in the progression of society will not be publicly recognized.