See http://msdn2.microsoft.com/en-us/library/ms178158.aspx

The main reason is that ntext fields can't be compared properly so queries with UNIONs fail (for example, the big query in get_my_courses in lib/datalib.php, you get an error message like "The ntext data type cannot be selected as DISTINCT because it is not comparable."). nvarchar(max) is new in MSSQL 2005, so it won't be compatible with earlier versions.