u/Cooper-Willis

Is there a reason why athematic verbs do not exhibit contraction in their primary mediopassive endings as thematic verbs do?

E.g. ´ιεσαι vs πειθηι or ´ιστασο vs ελαμβανου

Also why do the middle aorist endings of verbs like διδωμι and τιθημι contract but the endings of ´ιημι only contract in the middle imperative ´ου?