Right, so a group of people get to decide on who we can vote for, which leads to endless homogenization toward an agenda that we're not privy too. If politicians and other leaders weren't so obviously in it for themselves, caring about their own interests, Trump, Sanders, and a populist message would not have this platform.
Sure both parties would rather run the candidate their leaders think best represent the party's platform. Both parties ARE political parties and each does have a platform and does have rules that it sets up hoping to reflect not only its constituents' interests but the leadership's forward guidance. What's happening now in each party is instructive.
Look at the differences in reactions to Trump and to Sanders, respectively. Each major party is concerned about an excess of populism. Having formal ways of damping or diluting such a wave is a normal attribute in behavior of political parties in all representative democracies. The situations of the two parties are different and in turn their options and their chosen paths are different as well. Nonetheless the leadership of both parties strives to modify somewhat the surge of populism that has sprung from both left and right to support candidates in 2016.
Democrats: The reaction to Sanders is that the
platform of the preferred nominee is shifting a little towards that of Sanders. He may not get the nomination and there may or may not be movement of superdelegates towards him, but the party gets the message that their nominee's original stance on issues does not represent the core values of around half the party's activists (i.e. primary-voting members). Do not imagine that the progressive wing of the Democrats in Congress next term will be so docile as Pelosi and Reid have managed to keep them so far. That's even if who runs as the Democrat is a Hillary Clinton, the party-preferred nominee who was originally expected to represent more or less an extension of President Obama's policies as they evolved during his two terms in office. The superdelegates can ensure her nomination if they wish or believe they need to do so, but they will be standing up a different candidate than she was at the outset of the primary season.
Republicans: The reaction to Trump's overriding popularity in an originally crowded Republican field is that
the party is freaking out and imploding. There was
no preferred nominee in that huge field but there have been strong preferences for a traditional conservative nominee and for a hard-right conservative nominee, roughly reflecting Republican Congressional factions. Trump is actually neither of those. Trump is Trump, so far running as a populist under the Republican banner. He may or may not get the nomination but the potential electorate is getting the message that the party leadership does not accept that Trump represents the party's core values.
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Republicans are aghast that the dark side they've blithely courted for decades has re-energized some supporters so bigoted and hateful that even Trump has had to disavow. The leadership have not said they will try to broker a convention but they've pretty much indicated if Trump is their nominee then they will run someone else as a third party. That's pretty amazing for a party effectively
to choose defeat over endorsing a candidate they believe does not represent their core conservative values -- leaving aside the hot button positions they've used for so long to attract a large part of their base. The effect of the party leadership not supporting a popular nominee is unknown in modern times. But the party is basically telling some of its potential electorate to take a hike, which may result in a formal fracture of the party into at least two parties.
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Democratic leadership on the other hand are annoyed that their progressive wing has decided not to sit out another round of elections, having dutifully acceded to Pelosi and Reid's calls to get with the program all this time. But their approach so far has been to stick with the leadership-chosen nominee
while allowing a shift in policies outlined for the future. It remains to be seen what will happen if Sanders' appeal continues to rack up pledged delegates. The superdelegates are free to change their position as they see fit during the primary season. If they do not swing to Sanders then Clinton will be the nominee. It's likely that Sanders will then urge his delegates to get behind Clinton on the grounds that she has been caused to move far enough to the left during primary contests to improve chances for more progressive government if she wins the general election.
