The divine appointments of the Imams are decided by Allah just like it is in the case of the prophets! There is no elections or shura that takes place among people to de use who a prophet will be at the behest of people. Therefore to ensure the continuity of the message the ones tut comes after the prophet must be as infallible as the prophet himself in order that no deviation takes place after the prophet. And due to the face that this particular methodology was not followed after the prophet we see the kind of absolute mess and chaos the Muslim ummah is facing from then till now.