Your answer is grammatically correct, so it is not wrong, but it is not the best of the two answers. Here's why:
I did not know about it.
This establishes that we're talking about something in the past – a time when you did not know something. (Clearly, you know about it now, because you're talking about it!)
Now imagine that you say the second sentence as an answer to, "Why didn't you know about it?" And you want to say that you were not told about it. When should you have been told about it? Before you were expected to know, of course!
So if you "not knowing" is in the past, then the time when you were not told is the "past of past". For this reason, the past perfect is the better answer.