Because the truth is, and has been known for almost 3 years now. Why the resurgence in this tin foil hat conspiracy theory? Of course the most degrading thing is to the people who died on that flight. Lets see, they recovered proof that all passengers minus one were in the Pentagon damaged areas. But someone is managing to get more and more people to believe these people never existed, and something other then a hijacked plane slammed into the pentagon.

This conspiracy has been around long enough to be completly debunked many times over.

Why did the plane not make a plane shaped hole in the building? Well, because it was being smashed to bits starting the instant the thin metal of the plane touched a building built with layers of limestone, brick, cement, and steel. For a more in depth explaination of the damaage, I'll quote one of the subsites from the link above:

Paul: The question and photos are misleading: Parts of the plane penetrated the ground floors of the second and third rings of the building. These photos show only their intact roofs. Eyewitnesses and news reporters have talked about the twelve-foot hole punched through the inside wall of the second ring by one of the plane’s engines.

More importantly, the question focuses on the plane’s size and weight, making it sound extraordinarily heavy, but leaves out the size and weight of the Pentagon – America’s largest office building with three times the floor space of the Empire State Building - as well as the difference in relative stiffness and energy absorption between a building and an airplane. Each side of the Pentagon contains over 100,000 tons of Potomac sand mixed into the steel-reinforced concrete under its limestome facade. There are nearly 10,000 concrete piles anchoring each side of the building. And in the wake of bombings in Oklahoma City and Saudi Arabia, that portion of the Pentagon had just been reinforced with a computationally modeled lattice of steel tubes designed to prevent it from collapsing after an explosion.

By contrast, the plane is only 100 tons of custom alloys stretched thin enough to fly. It’s not like a giant bullet; more like a giant racing bike. Even so, the plane knocked down 10,000 tons of building material - 100 times its own weight - in the crash and subsequent collapse. Another 57,000 tons of the Pentagon were damaged badly enough to be torn down. The Brobdingnagian scale of the Pentagon makes the total area of damage seem small, but it would hold several Silicon Valley office buildings, or an airport terminal.

Patrick: Watch the videotapes of the planes hitting the World Trade Center. They were traveling at approximately 400 mph, and they hit an aluminum and glass building. An entire plane went in, and hardly anything came out the other side, 208 feet away.

Here we have a plane traveling at nearly 250 mph (just over 1/2 the velocity of the WTC planes, meaning just over 1/4 of their kinetic energy), hitting the ground (which would absorb much of that energy), and only then sliding at a much slower speed into a steel-and-kevlar-reinforced concrete and brick building. Obviously, it's not going to go very far. Still, parts of the plane penetrated into the C ring.