... because most Americans don't know who Gary Johnson is, and still won't by the end of the campaign.
Buzzword alert, buzzword alert! "Duverger's law." We have a plurality rule electoral system. Whoever gets the most votes wins. Now, for presidential elections, we have a convoluted way of aggregating votes to the state level, converting votes within a state to "electoral votes" and so forth, but it is all based on the plurality rule. Whenever you have a plurality rule system, there is a strong tendency towards two parties. That's Duverger's law, named after Maurice Duverger.
Gary Johnson is running as the Libertarian nominee, which is why he was too stoned in a recent interview to know what Aleppo is. But, that's OK, not just because most Americans are too uneducated to know where Aleppo is, but because most Americans won't bother to pay attention to minor party candidates. This is about as much attention as Gary Johnson will get.
Maybe Jill Stein can defend him by claiming that he just got vaccinated and suddenly developed autism...