That could also be because of the bias toward open-mindedness amongst liberals. Conservativism is defeated most effectively by the opening of the mind to new ideas, new thoughts, and differing opinions.
Conservatives are actually more aligned with "classical liberalism", though, so maybe the generalization is kinda right, but not like you intended.
... It seems to me, perhaps we have found one liberal who needs to be more open-minded and charitable with his intellectual opponents. Perhaps they have some right ideas and thoughts as well.