One can argue about it. You may have different unit preferences depending on the design. For example, when I do something for print I use cm, but when I'm making a web site I prefer to use pixels. As the units are stored with the document, every time I open, or even switch to the web design, I get units set to pixels, and back to centimeters when I open my design for print.
If the units setting was global, I would have to switch it manually every time I open different type of document.
In general, all the preferences that depend on the document type can not be global and are stored within the document.