Although both parties (Dem and Rep.) are late in the year (compared to the election date) this year, it is common in the modern history of American politics for the selection process to go all the way to the Convention floor, where, in many if not most cases, the nominee is decided by multiple rounds of voting as candidates are eliminated, until there is one clear winner left.

This hasn't been happening in the last couple of decades, but it was the way "the system" was designed to work, to have delegates select the final choice from among many nominees at the convention.