Why using static classes to share global data is not a good practice? [on hold]

I have heared that sharing data using static class properties is not a good practice. Although I haven't seen any one using this approach but I can't find out what are the drawbacks for such appraoch! To be more clear, Let's consider a WPF application composed of many UserControls that share data and parameters in a certain flow; using a static reference will make it easy to access/share these data, but no one seems to like this approach, Why?

I am expecting an anti-pattern related answer, I am just not sure what it is.