Goku has always been one to blindly rush into a new battle– the thrill of a new opponent is always too much for him. There are a number of times where this enthusiasm for a “good fight” results in disaster, and the whole Cell Junior debacle is a prime example.
Goku gets absolutely annihilated by the Cell Juniors and fails to make a serious dent in any of them. Goku’s defeat is made even worse here by the fact that it’s entirely due to his arrogance.
Goku enters this battle exhausted from his fight with Cell, and rather than take a Senzu bean to heal, he refuses it and gives it to Cell. It’s a move that completely backfires on him.
The Cell Juniors would have killed Goku if not for the help of the other Z Fighters. On that note, it’s Yamcha and Tien who provide cover for Goku and keep him safe in the end.