"What went wrong? You could point to Johnson’s missing the debates. He has lost about 1.5 percentage points from his national poll numbers since late September (when the first debate took place). However, he may have already been on a downward trajectory before the debates took place; on Sept. 25, the day before the first debate, he had 7.3 percent, on average, in national polls, compared with 9 percent a month before. So, it’s quite possible Johnson’s numbers would have continued to dip even if he appeared on the debate stage. Johnson, of course, has committed some policy-related gaffes — not knowing basic facts about the Syrian War or being able to name a foreign leader he admires — that suggest perhaps the debates would have been rough on him.
Another plausible explanation is that Johnson was simply a “protest” choice. Perhaps many voters who said they were going to vote for him weren’t really interested in Johnson specifically but were merely voicing frustration with Donald Trump and Hillary Clinton instead. There’s evidence for this. In August, when Johnson was flying high, a majority of voters had no opinion of him. In addition, many younger voters who as a group voted heavily for Sen. Bernie Sanders in the Democratic primary said they were going to vote for Johnson, even though Johnson and Sanders have very different ideologies. That seemed at least a little unsustainable. Indeed, as the campaign has taken shape and Sanders stumps for Clinton, Johnson’s numbers seem to be falling with young voters as Clinton’s rise."