Because the Founders never intended for the party system, such as we have now, to have taken root. The political system in this nation of states is rather like watching rival gangs or high school football teams, for that's about the mentality, competing. No matter who believes what, it's more of a game of king of the hill and 'we' have to be in control. You talk about the intent of the Senate and what it was to represent and not to represent. It is an absolute misconception to state that the Senate was not to represent the states.
Agreed, it was to be about the business of the national government, but with the concerns of the citizens from the respective states. For your argument to be valid, we may as well pull all Senators from one pool located in one place. But the Founders saw that, even with national business, the view of someone from Georgia may be different from the view of Delaware. Those views would shape the national policy. The intent of the system was not to play musical chairs or to rustle up a warm body just because they had the right letter by their name. But that's what it's become.
You can thank Jefferson and Madison for the creation of the modern political party as a national enterprize.
Examination of the Constitutional Convention notes verifies that my statement wrt the Senate being there to represent the permanent National interest is entirely correct. Just because it has been said over and over that it was there to represent the States does not change the facts. Now it is true that many Senators believed(and acted as if) they were there to represent States' interests but that was the secondary intention.
Senators could very well have been chosen from one pool since the Constitution does not forbid that. In fact, as early as the 1790s we saw the instance of Rufus King, a Mass. native, being appointed Senator from New York.
Why would one believe that only Senators were to represent States when Representatives were elected within the particular states as well? Why wouldn't they represent States just as well? Even the original method of Senatorial appointment points to the fact that they were analogous to the HoL and not elected by the People just as the HoL was not.
It is a common misconception of those who have not studied this issue extensively that Senators were to represent State interests. They are unaware that our government was set up with divided power to simulate the British structure: King- Pres., House of Representatives- House of Commons, Senate- House of Lords.