The Green party and the Libertarian Party got quite a bit of coverage during the last election but it was still minuscule percentage wise. There's really no incentive financial or otherwise to promote those who don't stand a chance. I think why we seen such a emphasis on the two groups last time might have been a push by the democrats to try and cut into Trumps numbers as they'd figured it was more likely people who chose to vote outside their political party would tend to be independents who otherwise end up swinging right.