Why the world trade center collapsed into its own footprint rather than having the top slide off?

No, you just have to take out enough supports that the rest of the supports can not hold up the building any more. For the top of the building to have slid sideways there would have had to be supports strong enough to support the entire weight while leaning to the side. But all the major damage was in a couple of floors and if the posts there had started tilting sideways (but there was no force sideways to get the upper part moving) after the tops had moved a foot or two the beam would start twisting and collapsing, thus dropping the building almost straight down. In fact, what happened is that the side supports for a number of columns failed in various ways and the beam that were supposed to stand straight began bowing sideways so more stress fell on the still straight ones bowing them until the failure rate rose. Once the millions of pounds of the upper building started moving the beams below it were taken to multiples of their design strength and failed.

You might want to look at the detailed computer simulation of building 7, which fell in roughly the same way after being on fire for hours with a bunch of conspiracy discussion there. But in fact once a few columns were heated by stored fuel and weakened (not melted) their failure transferred rapidly around the base overloading other beams and dropping the building. Here http://wtc.nist.gov/media/ScheuermanStatementDec20... and other NIST sites. Of course, there are dozens of sites sure that their speculations are more accurate than engineering and science.

http://en.wikipedia.org/wiki/Collapse_of_the_World...

http://www.nist.gov/public_affairs/factsheet/wtc_k...