Because he was a very good cheat.
He failed at least two though, apparently.
If he is honest and clean why is he not using his substantial wealth and already-appointed legal team to clear his name in open court?
Because USADA are self appointed judge, jury and executioner. The whole thing has been a stitch up from the start. A number of the witness they claim to have, have denied being suspended, as USADA states. The fact that USADA have allowed people who have, apparently, admitted to doping to continue cycling, stinks. And USADA have no jurisdiction to take anybody's titles of them. That is up to the governing body, but it hasn't stopped them trying. If I was Lance I'd be tempted not to fight it too. There's no point fighting a battle you know you can't win.