Question

Why should a thesis statement be significant, debatable, and limited?

Answer 1

If it is not significant, why should you want to write about the subject and why would anyone want to read what you wrote? You want to write something worth reading.
It should be debatable for much the same reason. If your thesis is "racism is bad", what could you possibly say that someone doesn't already believe? The very meaning of the word "thesis" implies "this is what i believe and I am prepared to defend it". There's not much to defend if everyone agrees with you before hand.
It should be limited, so you can thoroughly address the thesis in a reasonably short paper, as opposed to writing a 5-volume work on the subject! If the thesis statement is too broad, whatever you have to say in a brief paper will be too superficial.