Why the members of the boule chosen by lot instead of in an election?

The boule was an advisory citizen body of the Athenian democracy. Members had to be over 30 and citizens could serve on it twice, which was more than other elected offices. There were either 400 or 500 members of the boule, who were selected by lot in equal number by each of the ten tribes. In Aristotle's Constitution of Athens, he attributes to Draco a boule of 401 members, but Solon is generally taken as the one who started the boule, with 400.