Why does partisanship typically become a major obstacle in the impeachment process?

The impeachment process is prescribed in the Constitution. However, history has shown that partisan opposition from Congress is key to trial an impeachable offense.

James Buchanan, Ulysses S Grant, Harry Truman, and Ronald Reagan had sufficient backing in Congress to suffocate any attempts towards impeachment hearings even when there was evidence that there might be a reason to do so.

the second significant cause of the great migration was the desire of black southerners to escape segregation, known euphemistically as jim crow. rural african american southerners believed that segregation - and racism and prejudice against blacks - was significantly less intense in the north.