VW was willing, but Republicans weren't. Threats of loss of tax subsidies and unsubstantiated claims that the parent company said they would not expand if the union won overcame the neutrality of the company. Whether this was an unfair labor practice would appear to be a novel question, but one the union should test.
http://mobile.nytimes.com/2014/02/15/business/volkswagen-workers-reject-...
http://www.turnto10.com/story/24731630/uaw-falls-87-votes-short-of-major...