I work every day with SharePoint Designer, which I consider to be a great tool. But sometimes I discovered insert data view button is disabled.
If you ask why this is happening, because everything seems to be correct on the content page, I can tell you this is not a SharePoint Designer bug. It is related to ContentPlaceHolderID property. In SharePoint environment, this property is not case sensitive, so the code below is working, even if id of content placeholder is PlaceHolderMain, not PlaceholderMain.
<asp:Content runat="server" ContentPlaceHolderID="PlaceholderMain">
But even if your page is functional, not respecting case sensitivity of ContentPlaceHolderID property can create problems for SharePoint Designer. So, please be sure you respect this and adapt the code accordingly.
<asp:Content runat="server" ContentPlaceHolderID="PlaceHolderMain">