Why do the elements in group IIA have very low electron affinities?
The simple answer is reduced, effective nuclear charge.
With the Periodic Table of Elements, elements are arranged into Groups (columns) and rows (periods). Atoms decrease in size across a Period (from left to right as you're facing), but increase in size down a Group.
As a chemist, you should be able to rationalize these phenomena on a simple basis. You should also be able to rationalize the electron affinities of the alkali metals and alkaline earths, and compare these physical properties with those of the elements of Group VII. You should also be able to define "electron affinity".