This question might be answered more fully in the discussion section, but I will give my opinion. An employed person will have disposable income (by disposable I mean money which they can spare in other areas). Volunteering costs more than just time. It will cost the person financially through gas, contributions, and other financial areas. An unemployed person is likely to be seeking a job or have less resources with which to help others. It is also far more likely for an unemployed person to be depressed or unhappy with their situation. If a person is unhappy, they are less likely to volunteer their time with others. An a person is either unemployed because they cannot work or they cannot find work. A person who cannot work isn't likely to be able to volunteer. A person who cannot find work is more likely to spend their extra time looking for a job or some way to improve their personal situation.
I wanted to add to this, but did not see an edit button. I ask this question because you would think that a person who is employed in a fulltime position wouldn't have any other time to volunteer, especially if they had kids. This is just my thoughts on it.