why would the axel seal not be replaced at time of replacing axel?

It likely wasn’t replaced because there were no signs of leakage during the diagnostic inspection. MANY times the seal doesn’t get replaced when replacing the axle shaft. I suppose in retrospect- for this particular application - it would have been a good idea .. However at this point in time we now have the advantage of knowing the outcome and can easily criticize.. There is always the possibility the seal was damaged during the replacement procedure - that needs to be considered as well. Stuff happens!