Here's why:

* Cesium is an alkali metal, and alkali metals are known for their extreme softness.

* Cesium's softness is due to its weak metallic bonding. It has a single electron in its outermost shell, which is easily lost, leading to weak bonds between atoms.

* Cesium is so soft it can be cut with a knife at room temperature. It's actually softer than butter.

While cesium is the undisputed champion in terms of softness, it's important to remember that it's also highly reactive and dangerous to handle.