这篇来自Microsoft SharePoint Team Blog的博客很短，但是已经阐明了原因，引述如下：

With SharePoint Server 2013 embracing new web standards for client side rendering of pages such as JavaScript, JSON, and OData, there is no longer a need to support a visual web page editor within SharePoint Designer. With that in mind, we removed the ability to visually edit pages in SharePoint Designer 2013 because its page editor is designed to only understand the unique features of a SharePoint web page. With our support of new web standards, any web page designer can now be used for editing web pages in SharePoint Server 2013. This includes form customization, conditional formatting of page content, layout, theming and branding. To simplify the process of integrating customized SharePoint pages, SharePoint Server 2013 includes a new feature called the SharePoint Design Manager. This feature enables a web designer to export a web page from SharePoint, customize it, and then import it back into SharePoint, all right from the SharePoint site.

SharePoint Designer 2013 will continue to support site, workflow, list, library, and external data customization and configuration. However, we will look for opportunities to leverage SharePoint itself as the primary tool for customization and configuration tasks.

原文地址：

http://sharepoint.microsoft.com/blog/Lists/Posts/ViewPost.aspx?ID=1034