Why do some reflex actions involve interneurons, but some don't?
I did not realize it but the knee jerk reflex is a stretch reflex and thus only involves two neurons (monosynaptic). Many many more reflexes use 3 or more neurons (polysynaptic) so include interneurons. 

monosynaptic are common for muscle stretch limiting reflexes, that is why I knew them by their more common name stretch reflexes and did not make the connection. They don't involve interneurons becasue they would offer little benefit, they are self regulating most of the time and only involve a single source and a single destination and will not see much benefit from modification of the signal. 

This slideshare on reflexes can provide some detailed information and a place to keep researching from.