The difference between electronegativity between the C atom and the H atom is smaller than the difference between a CL atom and the H atom. And that's because the CL atom is much more electronic negative. That means it's going to pull the electrons toward itself. So the density will not be equal, whereas here it will be equal, or pretty much equal. Therefore, this section of the bond will be weak. And so when a base comes around, it will be able to pull away this H atom with greater ease.