Why fertilization occurs at ampulla isthamus junction?

IT IS A BIOLOGICAL NATURAL  REASON. BUT IT IS NOT NECESSARY THAT FERTILIZATION TAKES PLACE AT AMPULLA ISTHMIC JUNCTION ONLY.

SOMETIMES IT CAN HAPPEN IN SOME OTHER PART THEN UTERUS. BUT COMMONLY AMPULLA ISTHMIC JUNCTION IS THE MAIN SITE WHERE FERTILIZATION OCCURS. IT IS NATURAL AND I DO NOT THINK THERE IS ANY SCIENTIFIC REASON BEHIND THAT

REGARDS SAHIL

dear sehar 
yes , its not always nessasary that fertilisation occurs only at ampullary – isthmic junction , it can happen in other place . 
and this phenomemnon you can describe as ECTOPIC PREGNANCY . 
so, it happens there in general condition beacuse the temp and the condition in ampullary isthmic junction are suitable for fertilisation . 
hope it clears now 
regards 
kindly approve