Because a trademark does not represent an idea or an expression. A trademark represents you or a company. For someone to use your trademark, they are pretending to be YOU. For someone to use your idea, they are using information.
You still do not understand copyright, or intellectual property law.
A copyright does not protect information. It protects
expression. If I write a song, it is not "information." Information cannot be copyrighted, but the specific order of words used to express information (a textbook, for example) can be copyrighted. The theorems in trigonometry are information. They can't be copyrighted. If I write a trigonometry textbook, I can copyright it -- not because of the information conveyed, but because of the words I chose to express the idea. You can also write a trigonometry textbook. As long as you don't reproduce the same words I used in the same order I used them, you probably won't be infringing on my copyright.
Maybe photography is your bag. One of the most photographed lighthouses in the U.S. is at Portland Head, Maine. If I take a photo of Portland Head lighthouse at 5:47 p.m. on July 16, 2011, I can copyright that photo and sell prints of it. If you buy one of my prints and start selling reproductions of my photo, you are infringing on my copyright. You are taking money out of my pocket by selling MY image. However, you can set up your camera right next to mine in Portland at 5:47 p.m. on July 16, 2011, and take your own photo. It'll look an awful lot like mine, but it'll be yours -- and you can sell prints of your photo without infringing on my copyright. Why? The "information" contained in both photos is exactly the same. What's being copyrighted is not the information, it's the expression.