It takes governments years to get such things from design to operation. And major training programs are required. There are whole infrastructures already in existence necessary for such weapons systems to be developed.
This is nothing like 911 which essentially used the pre-existing strengths of Civilization against itself. Any future attacks will follow this pattern for the foreseeable future. It was an elegantly conceived endeavor costing little with staggering results.
There is little likelihood that the terror organizations are going to spend years and billions of dollars for a weapon which would be unlikely of achieving mass casualities. A plane of that size would not have a sufficiently large payload potential to justify it.