I'm sure we'll find out eventually, but I have a feeling it's because Yasopp didn't want Usopp to follow in his footsteps because he saw how much of a coward he was when he was younger, and didn't want him to get inspired to head out to sea and get himself killed. Probably figured if he never went back, Usopp would stay in the village, but didn't count on his son meeting Luffy and being pulled into the Pirate life.