Unfortunately, there's no easy way to accomplish this (unless there's a super-secret keystroke that I couldn't find). You could try placing a feature request for it here: https://www.adobe.com/cfusion/mmform/index.cfm?name=wishform
It's easy enough to delete unused styles by choosing "Select Unused Styles" from panel fly-out menu and then clicking on the Trash button. But reordering Styles is another matter.
The best suggestion I can offer is to export your styles as a library. To get them in the desired order, create a new file, open the Styles panel, and choose your original document from the dropdown menu. One by one, click on the style thumbnails in the order that you want them to appear. Then, reselect your Current Document from the dropdown menu, and you'll see the styles in their desired order. From here, you can export the styles as a library by choosing "Save Style Library" from the fly-out menu. Then return to your original document and choose "Import Style Library" from the Styles panel fly-out menu, browsing to the previously exported style library. Once imported, the library will appear within the Styles dropdown menu, and you can select that library and use it as the basis for future style applications within your document. (BTW, instead of exporting the entire document style library this way, you could opt instead to select specific thumbnails to generate meaningful sub-libraries.)
The Load Style… command would be another alternative to importing the style library, allowing you to bring the styles directly into your current document in the desired order. The problem is, there seems to be no way to accomplish this successfully without first deleting the previous styles, and doing this severs the connection to the objects on the canvas. In other words, if you wanted the objects on the canvas to show the correct style names, you'd need to reselect each object by hand and then re-apply the correct style. This would be a bit labor-intensive.
Perhaps someone else can come up with a more direct approach.