Why prefer factories over fixtures in Python testing?

From The Best of the Best Practices (BOBP) Guide for Python (https://gist.github.com/sloria/7001839), why should we prefer factories over fixtures? The link is to Factory Boy (https://github.com/FactoryBoy/factory_boy ) but I'm not clear on why, and in what scenarios, one should prefer factories over fixtures for tests. Is this always the case? When is it not?