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.
I've been in therapy for so long, I basically understand it well enough intellectually to be able to help others.
I know exactly how it's supposed to help me, but deriving actual benefit or being able to apply the techniques to myself has never been the slightest possibility.
i have to active catch myself having arguments with myself in the mirror, I think I'm Minority Reporting all the possible scenarios with the people ill talk to that day or thought of a better response later. its not healthy..but i did learn the brain is an organ that can be stressed like anything else , so, there's that.