I also think the white Jersey is a bit of a curse: Tejay in 2012 and Nairo in 2013 and 15, Pinot 2014. Puts loads of pressure on young riders.
Agree with Rich’s points about tactics, position and TTing. But also he’s a complete diesel and can’t kick. So while he’s hard to shake off, it’s very rare that he actually gains time by attacking.
Finally, in the Tour he usually has to deal with the ‘enemy within’ - Valverde seldom if ever rides for him, so that’s advantage all the other teams who have a clear leader.