Because of the situation of the stage (no MTF, with long descent and 10KM flat run in). Better to have someone else to work with and maximize the advantage. If he goes over alone, he is likely caught on the descent and all of the work to drop Andy is for naught.
As for not putting a stamp on this Tour, that's fair point. Last year, however, was a totally different course (opening TT, TTT and internal team strife), which warranted different tactics. I don't think AC or Astana has any interest in having the yellow jersey, or taking the weight of the race on their shoulders. Certainly they've shown their strength when it was advantageous (Stage 8 and 9), but they've otherwise been content to just minimize their effort where possible.
More to the point, Andy HAS to put his stamp on this Tour much sooner principally because of his relative weakness in the final TT. He needs (or needed) to gain as much time as possible on his rivals in advance of Stage 19. AC doesn't have that same impetus and cane be more selective about when he attacks. So I don't know if you can draw any conclusions about AC's form because he has not elected to attack.