I suspect that it is related to the fact that Senators were originally elected by the state legislators and not diretly elected by the people. This resembled the House of Lords in England's Parliament where membership was heriditary and for life (our senators have tenure for 6 years, our representatives only 2). This was intended to free them from the pressures of running for office and enable them to bring the business of the states to the Congress. Their function is quite different from that of the house, particularly in regards to economic legislation.
A little research into the Seventeenth Amendment will give you the entire history of why this was changed.
Just have to clarify - there are 435 in the House, not 536.
The 2 year House terms are designed to make Representatives be close to the people. They are basically always running for re-election, and the entire House is up for re-election at the same time.
The Senate not only has 6 year terms, but only 1/3 of the Senate is up for re-election at a time.
Also, Senate seats are state-wide offices, where as House members generally represent a much smaller constituency.
In addition to the previous comments, senators are elected for 6 yr terms vs. Representatives 2 yr. terms. So not only are there fewer Senators 100 vs. 536, but they serve longer.
The Senate has certain powers/authorities that the House of Representatives does not.
1. Senate approves presidential appointments (justices/ambassadors). The House does not have any say in these areas.
2. The Senate approves treaties that the President creates (the House has no say in this area.
3. The Senate has 2 representatives from each state and therefore is considered an equal representative system of the 50 states.
4. If the President is impeached by the House, the Senate decides the fate of that President (either found guilty or convicted and therefore removed from office). In other words, the Senate can remove a President from office.
5. Generally Senators often have served in the House of Representatives or state governments. States have multiple Representatives, based on their representatives but only TWO Senators, regardless of population. In other words, in most states, there are fewer Senators than there are Representatives