Why?
1. "President" Bush just cut funding to NASA
2. There is no cold war competition to drive an expedition out of a profound feeling of nationalism
3. There is no public support for a trip to the Moon or Mars, because the American people are so stupid that they would rather put the money elsewhere, and don't see a reason to go there
4. The costs of going to Mars are astronomical because a new shuttle would have to be designed, and the new shuttle was just cancelled, not to mention that the technology hasn't increased enough, such as growing food, finding a way to reach the surface without dying, surviving the martian heat for a long time, etc.
a counter question:
If the technology to fake a moon landing has increased so much, making it ridiculously easy, then why haven't they faked one again?