The description of the issues/requirements is not included by default in an excel export, because this field usually contains a long description of the problem, requirement which hardly fits to an excel cell (Excel has serious limitations what a cell can contain, like it cannot display a real rich content in a cell, and a cell can contain only up to 32000 characters).

However because usually the description of an item contains the most information about the subject you have the option to export this to the excel field, by turning on the Export description check box on the excel export dialog.

For limitations of exporting description, see Field Length Limitation in the topic :