Widespread anti-Semitism among the general non-Jewish population made it really difficult for Jews to obtain weapons and the materials needed to support an armed resistance. Without that support, any sort of organized resistance was unfeasible. This widespread anti-Semitism was pretty common throughout all of western Europe at the time and also helps explain why German Jews didn't just pick up and move elsewhere in Europe when the Nazis first came to power.   


In addition to that, the Nazis made it a point to punish any efforts of armed resistance by rounding up a disproportionately large number of Jews and torturing them to death (the Warsaw Ghetto resistance offers an idea of how the Nazis implemented this). I can't even imagine what it would be like to be stuck in that dilemma of needing to fight back to survive and knowing that your actions will ultimately lead to hundreds or even thousands of other people being subjected to some of the most mind-wrenching torture techniques until they eventually die.