A theory applicable is Adverse selection. From an economic perspective we might expect that freedom of information legislation to encourage transparency, however, there is every likelihood of market failure and the adverse outcome results with more procrastination and concealment of data. A strong ombudsman presence helps and judicial review if needed. This at least does seem to work in New Zealand where we have had OI release for a long time.
Data trawling by journalist and those looking for competitive advantages makes the process very expensive and time consuming for those who receive lots of requests. Logically such organisations will be frugal with resources and may only release the minimum amount to cover a query. The more specific the query the more specific the response but general queries looking for a lead tend to receive non-helpful responses.
Good luck with your research. Google OIA NZ if you are interested material from here.
In addition to the reply by Mr. Locke, the weak public governance structures are another reason for this. In many poor and developing countries, the governments are not true representative of the general population, or corrupt and defy any system of check & balance or accountability. In these situations, it would be in their best interest to be secretive about data revealing their corruption, inefficiency and ineptness.