A man operating a silver 2002 Toyota Camry approached the restricted area twice, though never accessed the plant, officials said.
So this sounds like he was inside the perimeter but potentially not within the boundaries of a higher security interior zone, to me. It may not mean that at all, but if it does it might explain (see my comment above) that they weren't authorized to use force under the circumstances.
Again, I recognize this is just speculation on my part, just explaining my thought process.
I would guess complacency initially (can't recall a similar incident in recent memory), and also some of the language in the article lead me to believe that a particular threshold wasn't crossed. Maybe that threshold is the only level of threat under which they are allowed to respond with deadly force. (that's just rampant speculation on my part)
I get the impression they were private security not police, so that would explain them being less trigger happy.