Answer:
Their gravitational pull is too weak.
Step-by-step explanation:
Earth, Mars and Venus are terrestrial planets that used to have hydrogen and helium in their atmospheres. However, these light elements were lost over time as they evaporated. This is primarily due to the planets' having weak gravitational pull when compared to the other bigger planets like Jupiter, Saturn and Uranus and the planets' warmer temperature that results to having a thin atmosphere.
On the other hand, the bigger planets have large masses and are cold enough to prevent both hydrogen and helium molecules from escaping the atmosphere.