Why did Republicans in Congress want to impeach Andrew Johnson?

The Radical Republicans in Congress felt that Andrew Johnson, a southerner, was too easy on the south during Reconstruction. The Republicans in Congress, such as Thaddeus Stevens and Charles Sumner, wanted to radically reform the south to abolish the old planter class, while Johnson's plan for Reconstruction was more lenient.
The Republicans watched as the Black Codes were instituted in states such as Mississippi; these laws prevented the free movement of former slaves and abridged some of their other civil rights. In response, the Republicans wanted to pass the Civil Rights Act of 1866, but Johnson vetoed it, saying that it infringed on states' rights. In addition, in 1866, Johnson vetoed the bill to renew chartering for the Freedmen's Bureau (founded in 1865), which had provided assistance to former slaves such as finding family members, education, and the promotion of fair labor contracts between former slaves and plantation owners. Johnson also dissuaded states from passing the 14th Amendment, which gave citizenship and due process rights to African-American people.
As a result of these feuds, the Congress passed the Tenure of Office Act in 1867, which limited Johnson's power to remove cabinet members. When he removed Secretary of War Stanton, who was a member of the Radical Republicans, the House moved to have Johnson impeached. He was saved by conviction in the Senate by a single vote.