In effect it's because the lower house is meant to be radical while the upper house is meant to be conservative.
Both are meant to balance each other out.
Well yes I see what you are getting at but you are taking a someone different meaning of "radical" and "conservative" than the way these terms are used in the contemporary US.
The House was meant to be reflective of short-term popular will- the means to press for urgent and emotive actions that the people demanded.
The Senate was meant to be the voice of reason and sanity where cooler heads prevailed and experts not concerned with immediate re-election worries could consider the long-term good of the United States.
There is no specific reason to associate the first tendency (expression of short-term popular will with liberals/ democrats) and the second (sane and considered action) with conservatives/ republicans.
If you consider the traditional meaning of RADICAL (connected to extremism-pushing for vast changes) then the house is certainly more radical. But that applies to BOTH DEMOCRATS AND REPUBLICANS. While house democrats are more likely to push (RADICAL LIBERAL Concepts) like the single payer healthcare scheme, house republicans are likely to push (RADICAL CONSERVATIVE concepts) like privatization of social security.
Senators of both parties less LIKELY on average to push for either schemes.
So yes the function of both houses as conceived by the founding fathers is being fulfilled but I must respectfully disagree that the nature of one house favors the election of liberals/dems and the other favors elections of conservatives/reps.
One may favor radicals/extremists (House) while the other may favor moderates (Senate) but either of these can be found in both the two parties and ideologies.