The electoral college, created to give power to slave states, is enshrined in the Constitution. The details of political nominations are not. The nominating process has evolved over the years:

• King Caucus–Candidates were selected at party caucuses. Delegates were appointed by state legislatures. The people had no role. Around 1835, when Andrew Jackson was President, the legislative caucus was replaced with a national convention.
• Conventions–Since 1836, every major party has nominated its candidate at a national convention. Conventions eventually required delegates to take a loyalty pledge.
• Primaries–Primaries gave a role to the voters, but initially primary victories didn’t matter. In 1952 Estes Kefauver won twelve primanries; Adlai Stevenson, the nominee, entered none. Robert Taft won six while Eisenhower won five; Eisenhower was nevertheless the nominee. However, since 1968, the winner of primaries has been the nominee.
• Super delegates–In 1984, Democrats, recognizing the danger of an unsatisfactory candidate winning primaries, created a system of super-delegates to return some discretion to party officials. Republicans, sadly, have yet to do this. After Trump, they may take this precaution.

Candidates Sanders and Trump claim that the system for Presidential nominations is flawed. Both have a point. No completely satisfactory process has yet evolved.

For an interesting discussion see this article titled “How to steal an election,” by historian Jill Lepore:

http://www.newyorker.com/magazine/2016/07/04/conventions-primaries-and-the-presidency