That one electron in the outermost shell has two things going against it - first it is only one electron, and second is its distance from the nucleus.
As we know, electrons are usually sorted into neatly arranged orbitals that contain specific preferred numbers of electrons, like
Then there is the great distance the electron resides away from the nucleus, resulting in a very low electromagnetic force between them. That means Cesium is the most electropositive element, and is highly reactive. This property make it useful in photoelectric cells.
There is plenty of information about Cesium here: