Avraham is described in the midrashim as being extreme in the mitzva of hachnasat orchim (hospitality). Through this method, he was able to mekarev (reach out to) thousands of people and teach them about God.
I know of no parallel mention of Yitzchak or Yaakov continuing this huge hospitality program, so it seems it stopped with Yitzchak. Why didn't Yitzchak follow in his father's footsteps regarding this? Did he perhaps continue it with a different method?