Comments don’t affect the processing of an XML document.
With XML, that might not be the case. For example, the content might consist of numeric data that a human can’t easily interpret visually. When working in CSS, it’s not easy to add explanatory text when rendering the XML document. This limitation might not be important when you’re working with documents that contain only text, but it might be a big consideration when you’re working with other types of content. Another limitation of CSS is that it mostly renders elements in the order in which they appear in the XML document.