Different applications understand data differently, even the same kind of data.
For example most word processors support some form of formatted text with different fonts, styles, sizes and so on. However Microsoft Word 6.0 for the Mac is not able to read formatted text produced by ClarisWorks and vice versa. Pictures from Photoshop can be copied and pasted into many programs, but the layers tend to get flattened when you do that. The clipboard only really understands raw bytes. It does not inherently know how whether those bytes are text or a picture or a table or something else.