I would suggest that it has more to do with a lack of funding for 3rd parties as well as a lack of truly top tier candidates. Look at 1992 when Ross Perot spent a pile of cash and nearly got a viable 3rd party up and running. They were included in the presidential debates, held some congressional seats and even a governorship or two. But as they failed to continue finding good charismatic candidates the bit of money that was there dried up and by 2002 the Independece party was pretty much a thing of the past.
Find a source of money and a good spokesperson/leader and you'll have a 3rd party.