Why is the number of members in the House of Representatives based on state population and the Senate is two per state?
A balance of power.
In making laws, it takes an agreeing vote by both chambers, senate and house, to get the law passed. To give each state an equal footing regardless of its population, two senate seats was incorporated. The house of representatives enumeration helped establish the 10 year requirement for a census. After each census the 435 seats available are distributed according to population.
Now, if you only had the house of representatives and no senate, then large states would virtually control all legislation. The structure of the senate evens the playing field.