there are fewer members of Congress in the Senate as each state only gets exactly 2 senators while the house of reps is based on population and currently has 435 members. this makes the Senate more powerful and harder to get into.
senate terms are also 6 years while house of reps terms are only 2 years. these are only a number of factors making the senate more desirable to be in for someone seeking a position of power, leading to stricter regulation for candidates to win a seat in the senate