Because we never know when an event will take place. But I think a better question is "Why not"? Why can a person with a CPL (or anyone else) be deemed OK to carry everywhere else but a few select places?
If we can, for the sake of the argument, accept the ban on CC by CPL holders in a school, why is there no allowance for concealed carry with permission? So, even if a school wanted to have some staff or even visitors conceal their pistol, it is prohibited unless the CCer is hired as security pursuant to a contract requiring the person to carry a concealed firearm.
In closing my little diatribe, I believe that the exemption for people who are not LEOs acting in the course of their employment violates the equal protection clause, much like California's "assault weapon" (sic) registration scheme exemption for retired LEOs was found to be unconstitutional on equal protection grounds. Having been a reserve officer, an exemption for individuals who are civilians with no police powers and no legal requirement for firearm training beyond the CPL is ripe for a legal challenge on equal protection grounds.
