Because the Senate rules say so.  Because all 100 Senators typically attend the impeachment trial, especially one involving the impeachment of a president.  Because impeachment must be expedited due to possible threat to country form president/official being impeached. Do you really think anyone would watch Grahams FISA hearings if an impeachment trial is being held on the same day?  They found a way to delay the process a few days(edit: probably more like weeks/to months depending what McConnel has to do to get the articles of impeachment) while they try to find a way out. But there is no way out.  Edit: McConnel could be brave and just start an impeachment trial without them but is McConnel a real patriot.  The Senate would likely have to vote to change the rules of impeachment governing the Senate.