Because there are 50 states. When there were 48 states the Senate had 96 members. The constitution sets 2 Senators for each state, to allow equal representation among the states in the federal government. 2 were determined in the event a Senator could not finish a term, or was absent during a vote. The state would still have at least 1 representative. Since regardless of the number of states, the number of Senators would always be an even number, the Vice President serves as a tie breaker in the Senate.
50 states and each states to have two senators = 100 senators. Representatives are different is depending on the number of people in each district. When the population grow, the district devide to have more representative. For ex, Texas has grew so much by the census and they need to aquire 5 more representatives in 2010.
The United States each state have two members in the Senate as a representative because the U.S. Has 50 states.