What is the writer's purpose in excluding only Trevor's limited omniscient point of view?
You have asked an excellent question. Of course, the novel as a whole is written using the omniscient narrator - it is not told from the third person limited point of view as we have access into the thoughts of a number of characters, as you have noted.
To me, the reason why Greene excludes us from the thoughts of T. is because it forces us to infer his motives and his state of mind from how other characters view him, his actions and his speech. In a sense, having his profoundly disturbed mind revealed in this oblique way is an excellent narrative strategy in that it forces us to work to make sense of him as a character. It also shows us how extreme he is as well because of the differences between him and the other gang members such as Blackie. We are forced to try and piece together the fragments of what we are revealed about him just as surely as he is so focussed on his act of destruction. Consider the following quote:
"Of course I don't hate him," T. said. "There'd be no fun if I hated him." The last burning note illuminated his brooding face. "All this hate and love," he said, "it's soft, it's hooey. There's only things, Blackie," and he looked round the room crowded with the unfamiliar shadows of half things, broken things, former things.
Think of what this shows about T. He is completely detached from "normal" human emotions. He neither hates nor loves and he destroys without passion or attachment. In a sense, by excluding us from his thoughts, we as readers become spectators of T., his actions and motivations, as we struggle to understand what could have happened to make a boy so profoundly nihilistic.