What does Genesis 24:9 mean?

Abraham has ordered his servant to find a wife for Isaac, among Abraham's kinsmen back in Mesopotamia. However, Isaac is not to leave Canaan—the woman must be brought here. The servant is concerned he might not be able to find a suitable woman and convince her to travel so far to marry Isaac. So, Abraham made it clear to his servant that this oath would not be binding if circumstances were beyond his control.

With that reassurance, as Abraham had requested, the servant would swear not to allow Isaac to marry any local Canaanite woman and would, instead, travel himself to the land of Abraham's people to attempt to find a wife for Isaac who would agree to marry him and live in Canaan. The servant put his hand under Abraham's thigh, as was the custom, and took the oath. This symbolic gesture implied that the oath-taker would be at the mercy of Abraham's descendants if he did not fulfill his end of the bargain.

The whole conversation raises an obvious question: Where was Isaac? As a man around the age of 40 (Genesis 25:20), why did he not participate in this conversation? Why was Abraham's servant given so much authority over Isaac's decisions instead of Isaac himself being included?

We're never told. Perhaps custom demanded that finding a wife was absolutely a father's duty and one Abraham could not fulfill without the help of his servant. Perhaps Isaac was a particularly passive person. In any case, Abraham saw it as his responsibility to find his son a wife without, apparently, Isaac's input.