Most likely because you are viewing a PDF in an application that does not support annotating PDFs. Many Android applications do not support annotating PDFs. Applications that support annotating PDFs include PDF Reader Pro, Acrobat Reader, and more.
If you want to make sure your annotations are visible on Android, you can save a flattened copy of the document. The flattened copy's annotations are no longer editable but can be seen in any PDF viewer.