The importance of statistical transparency is part of the reason why I've switched to GitHub and Jupyter notebooks, as GitHub renders them natively which makes it obvious what code is being run and what the results of the code are. (two recent examples of mine are processing AngelList data [https://github.com/minimaxir/sfba-compensation/blob/master/a...] and creating graph networks from Reddit data [https://github.com/minimaxir/reddit-graph/blob/master/subred... ])

That said, there are a few reasons I've seen writers/researchers intentionally not release data. Either the code is sloppy/inefficient/embarrasing, or the data is used in bad content marketing for a startup that specializes in data collection so that they can say "we have data, pay us if you want more, neener neener neener." (The latter of which I see submitted to HN all the time and serves as a pet peeve of mine)