SharePoint: Explorer View option is not available on the Current View list box

After a recent upgrade from SharePoint 2007 to 2010, users find that Explorer View is no longer available on the View list box on the Ribbon. 

In SharePoint 2007, the view was available in the View list box:

1. Access a Document Library on 2007.
2. On the top right hand side of the Document Library, click the View list box.
3. Note that Explorer View is available.

In SharePoint 2010, the customer is trying to find Explorer View in the same list box:

1. Access a Document Library on 2010.
2. On the Ribbon, select the Library tab.
3. In the Manage Views section, click on the Current View list box.
4. The customer expects Explorer View to appear in this list.