Not sure about the role of New York values, but in any case, that was the liberal party line, then and now. Shortly after the assassination, Warren Leslie contended in his book Dallas Public and Private: Aspects of an American City (New York: Grossman, 1964) that the climate of "right-wing extremist" and "hatred" that pervaded Dallas primed the city to be the site of JFK's assassination. That party line has been maintained in Dallas 1963 by Bill Minutaglio and Steven L. Davis (New York: Twelve, 2013.
For an excellent analysis of how the party line was formulated, read Camelot and the Cultural Revolution: How the Assassination of John F. Kennedy Shattered American Liberalism by James Piereson (New York: Encounter, 2007)
Good stuff.
Thanks.
I will check these out.