Because the USSS and the State Dept. are packed with Clinton holdouts who have a vested interest in disrupting the Bush Admin. and the GWOT, and if someone in the Administration gets whacked, then it's a bonus for them.
All of the kiss-and-tell books about "former SS agents" and "former WH aides" saying how hated the Clintons were by the staff and support, holds the same water as Scott McClellan. Which is to say, none. The WH staff is full of Bill Clinton loyalists who are simply marking time until next year, and a third Clinton term.