Question

why is it considered a bad thing to oversimplify things?

I would say that oversimplification is harmful as it can lead to error & cause one to undermine the importance & complexity of important issues.

But I need some more ideas?

Answer 1

You might explain why and how oversimplification can lead to error. Otherwise, I think you have it covered. The error is often that the issue is distorted and/or no longer accurate.

http://dictionary.cambridge.org/us/dictionary/english/oversimplify