Johnson is polling at 10% nationally, with about 2.3% chances of getting one single Electoral Vote, let alone 270. So, with a winner-takes-all electoral system, voting for Johnson is akin to not voting, or voting for the Spaghetti Monster- a waste of a good vote.
Also, Johnson's positions including cutting the budget by 43%, cut corporation tax to 0% and replace income taxes with a 23% sales taxes (who hit poorer people more, as they spend a larger share of their income in consumption);
supports displays of Confederate flags, abolish the Department of Education, end public education and instead give vouchers for young kids to go to basic and high school, and ban public loands for college students (tuition? Hope you have deep pockets!);
wants to remove all regulation about energy, do nothing about climate change, fracking is good;
wants to leave parents to choose whether to protect their kids from vaccinable diseases;
is against all tariffs or trade restrictions of any kind;
wants to completely privatize health care (aka kill Medicare, Medicaid and any sorts of subsides, if you can't pay out of pocket, you are free to die on the streets);
supports cutting unemployment benefits, raising retirement age to 75 and privatizing Social Security;
wants to give no federal funds for a public mass-transit system, but build more highways for private cars instead;
https://www.ontheissues.org/Gary_Johnson.htm
So yeah. You know Bernie Sanders? Then picture the opposite in everything except in legalising drugs.