Not because I didn’t care about what had happened, but because the enormity of it meant it was simply too difficult to immediately process. 

【翻译】不是因为我不关心所发生的事情，而是因为它的严重性意味着很难立即处理。

动词 mean 表示“意味着；意思是”，是及物动词，可以接that 引导的宾语从句。而that 引导宾语从句时，常常省略。本句也不例外。