It seems they rejected it because it was "too right-wing" yet we would have rejected it because it was "too socialist" for the Anglo-Saxon world...
Yes, many French on the political left see the EU constitution as too "liberal" in the classic sense of liberalism--free markets and abolition of tariffs across national borders as capital moves freely within the EU member countries. To a socialist, classical liberalism is right wing.
To others on what is considered the political right of French politics, they see the constitution as a threat to national sovereignty and all things French. So there is this strange and informal agreement between the nationalists on the right and the communists on the left to reject the constitution.