The biggest difference between the two parties lately is that the Democratic Party chooses candidates based on policy/ideology, while the Republican Party chooses based on who will win.
Reagan used to be a Democrat? Who cares, he's popular. Trump used to be a Democrat? Who cares, he's popular. The Tea Party is openly opposed to the current leadership of the party? Who cares, they're popular.
Republicans prefer to win first, then negotiate within the party to decide what to do. Democrats have the negotiation first, then try to win with the "best" candidate.
Bernie didn't support the Democratic party platform. They don't have to let him run under their banner.
Reagan used to be a Democrat? Who cares, he's popular. Trump used to be a Democrat? Who cares, he's popular. The Tea Party is openly opposed to the current leadership of the party? Who cares, they're popular.
Republicans prefer to win first, then negotiate within the party to decide what to do. Democrats have the negotiation first, then try to win with the "best" candidate.
Bernie didn't support the Democratic party platform. They don't have to let him run under their banner.