This happened in the Democratic Party in the early 70's when George McGovern won the nomination - and was subsequently trounced by Richard Nixon in one of the largest landslides in American Presidential History. In the last election, the Republican Party Leadership had strong reservations about John McCain's candidacy. That would never happen in European Democracies. However, it is probable that the last two reasons are the most important. These two factions coalesced into our first two political parties.
Politicians write the rules to serve their own interests.
And if individual politicians do so, so do political parties. The Democratic and Republican parties set a number of rules at their state and national legislatures that intentionally stack the deck against the emergence of third parties. They are reinforced in this effort by the media and the US educational system. These rule may include a number of signatures or amount of money which must be amassed by a particular date to qualify. The higher the Democratic and Republican state legislatures set the bar, the more difficult it is for third party candidates to get on the ballot.