First, I would recommend to double check the internal name of your field, just in case.
do you access the data retrieved like this ?
function onSuccess(data, request)
var itemInfo = data.d;
// if your rich text field internal name is "Description"
if your field is not here, can you try to modify the rest URL by adding
at the end. See if you can retrieve Title and your field.
It doesn't seem possible to retrieve the PublishingPageContent via REST in SharePoint 2010.
It is in SharePoint 2013 :
How to get content of html by using REST service in SharePoint 2013