At first it’s hard to understand what swinging a chicken over your head has to do with Yom Kippur the holiest day of the year when we fast and pray for atonement for our sins. We can’t gain forgiveness for our sins without teshuvah repentance including those sins which carry the death penalty. By being allowed to substitute the death of a chicken for our own death (realizing that what happens to it should have happened to us) one will (hopefully) appreciate Hashem’s mercy and be inspired to repent. Why doesn’t everyone do kapparos? There are several reasons. Of course many of our fellow Jews simply never heard about it; or if they have don’t understand it. However the poskim have argued over whether kapparos are a good idea halachically. Some were against it because it looked too much like certain non-Jewish customs or that people would mistakenly think that the chicken’s death frees them from doing teshuvah. But since many great rabbis upheld the minhag it remains common in Jewish communities today.