Because you’re focusing on wanting to not think about it rather than focusing on observing your thoughts and details about the present moment. Thoughts will come and go, allow them to, then gently redirect yourself back to something like breathing, physical sensations, etc.

That’s the gist of what’s been kind of helping me the most of everything I’ve tried and it’s quite difficult to practice and there’s a lot more that goes into it (like you do have to actually challenge some thoughts) but yeah.