I'm getting tired of people using Democracy and Republic interchangeably. And I hope hope you'll believe me davydagger when i say I'm genuinely not trying to troll you so forgive me if I'm inferring more from your post than I should be.
The two terms don't mean the same thing. The US was not created to be a Democracy (and Rome was never a Democracy). Democracies have tendency to degrade into tyrannies is the main reason. As soon as a populace realizes they can vote themselves lots of free money along with that famous "51% can be pee on the cornflakes of 49% of the population" line it's just a slow downward spiral (I believe this is not irreversible though. Nothing is set in stone or inevitable). I don't know who said that originally. Republics are a different thing. They're supposed to have safeguards to prevent or at least slow the slide into tyranny.
I happen to live in California where the flag specifically states Republic in no uncertain terms (by which I mean the flag has the words "CALIFORNIA REPUBLIC in big bold letters). As in elect representatives and they do the government running part full time. In a democracy 100% of population would vote on 100% of the matters affecting the population. How long would that last?
Here's the part of the US constitution, if you're wondering. The entirety of Article 4 is actually really short. The Constitution is an amazingly brilliant document.
Article 4, Section 4
The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence.
You'll note it doesn't what kind of republic the states must be. Louisanna for instance is based on the "Napoleonic Code" while the rest are based on british common law.
Also, the house was supposed to be "for the people" as in something happens (like a terrorist attack) and your average citizen talks to their representative which causes the house to pass a bill "in the heat of the moment". The Senators on the other hand, not nearly as much worried about an upcoming election, can take a slower approach. The idea being the bill either won't get through the senate or at least a more moderate version will eventually emerge. I don't know if that's really the way it's working for us right now. Would the Patriot Act be more radical if not for that house/senate balance? No idea. Point is, that was the idea.