The immediate reason was because of Bilbo's smell. Gandalf says that Smaug would not recognise a hobbit scent, and also hobbits are naturally quiet. He chose Bilbo because he felt that of all the hobbits he would be the most eager for adventure.
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Utúlie'n aurë! Aiya Eldalië ar Atanatári, utúlie'n aurë! The day has come! Behold people of the Eldar and Fathers of Men, the day has come!
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