There's a 1973 DOJ memo that explains the rationale
A necessity to defend a criminal trial and to attend court in connection with it … would interfere with the President’s unique official duties, most of which cannot be performed by anyone else. It might be suggested that the same is true with the defense of impeachment proceedings; but this is a risk expressly contemplated by the Constitution, and is a necessary incident of the impeachment process. The Constitutional Convention was aware of this problem but rejected a proposal that the President should be suspended upon impeachment by the House until acquitted by the Senate. …
To wound him by a criminal proceeding is to hamstring the operation of the whole governmental apparatus, both in foreign and domestic affairs. It is not to be forgotten that the modern Presidency, under whatever party, has had to assume a leadership role undreamed of in the eighteenth and early nineteenth centuries.