I feel that when 3rd party candidates have brought something to the table, they've been acknowledged and given the space to make their case. George Wallace, John Anderson, Ross Perot, Ralph Nader, Gary Johnson are examples of 3rd party candidates who were heard -- but eventually rejected. Each one had enough publicity to get noticed and if they were better candidates could have been viable candidates. Given the length of time in the presidential election process, and given the wildly available access points in the media, I think the primary reason that 3rd party candidates haven't risen to equal footing with the Dem and Rep candidates is at least in part due to ideologies that people aren't buying into and/or a lack of capability by the candidate him/herself to grow attention and support.
There was a time in the last election cycle that I was considering Johnson-Weld -- not at all because I lean toward libertarian ideology (I don't at all) -- but because I wasn't enamored enough with Hillary and couldn't stand Donald. I watched, I listened, and in the end I just thought Johnson was an awful candidate (seems like a good guy thought). Had Weld been the nominee I might have gone in that direction.