One reason could be explosive devices. If explosive devices were able to get close enough to the buildings like they did in 93 it would have been a proven security breach of exactly what security was suppose to be watching for. This would have caused an insurance payout nightmare as I’m sure all kinds of security measures had to be met after 93 in order to reinsure the buildings. It also would have presented a liability problem for the Port Authority. The plane and plane initiated fires make it no fault and a much easier insurance payout. Even with this no fault claim it still took 5 years for Silverstein to collect his money. This covers the insurance claims as well as getting people off the hook whose job it was to watch for explosives getting near the building.
The following article proves that they would be concerned with just that…
http://findarticles.com/p/articles/mi_qn4189/is_20010914/ai_n10164926
Another possibility I think is that its possible WTC7 was never meant to be damaged but once it was it posed a security risk. It was a high security building that housed several Gov agencies and once it was damaged it could no longer be safely secured nor possible to safely retrieve the many sensitive documents and data inside. A decision had to quickly be made as to what was the best thing to do. So they pulled it.
Or it could simply be something only a real investigation could uncover.