Why do impacting electrons have a low probability of causing ionization?
Ionization has a low probability of occurring if the energy of the incoming electron is exactly equal to the ionization energy. Using a classical analogy, if  ionization occurred the incoming electron would be left with no energy, ie, stationary. The incoming, now stationary, electron would not scatter away but rather remain trapped by the nuclear attraction, such that no net ionization would have occurred. It would also imply (classically) a colinear collision with the departing electron, a seemingly unlikely event. On the other hand, if the incoming electron has a significant energy surplus, such that it has sufficient energy to escape the nuclear attraction (scatter), then ionization is more likely to occur. 

There are obviously more accurate non-classical ways to describe the process, but the analogy illustrates the problem. There are also intermediate scenarios, such as those exploited in Auger spectroscopy.