Why did Duccio paint such a complicated altarpiece for cathedral of Sienna?

This is a History essay, with a large focus on art. Italian nationals and indeed enthusiasts in Gothic art and architecture from the 13th-14th century would be needed. I expect no scrambled english or ‘essay bot’ tendencies in this essay. Or I will swiftly find a refund. This link contains plenty of Primairy source material to cite in HARVARD prereably.