Hello Jef Vercammen,
You can create a duplicate of the dimension field that you need to alias. That duplicated field does give you the alias option when you right click. This is a workaround.
There are a wide variety of reasons that a field may not be able to be assigned an alias. However, in this case, I believe that the cause is a published data source. When connected to a published data source, aliases cannot be edited. If the alias were to be edited, other workbooks attached to that data source could become corrupted, fields and calculation could break, etc. Accordingly, Tableau Desktop 9.0 does not allow the user to edit aliases on published data sources.
There was a bug in Tableau Desktop 8 for some time that allowed the aliases of fields in published data sources to be edited, but such a bug does not exist in Tableau Desktop 9.0. Accordingly, the desired editing cannot be performed.
If the alias of a field in a published data source must be edited, please first download the extracted data source that has been published. Next, please edit the alias as necessary, then republish the extract.
Feel free to upvote this idea from the community! Edit Alias of table calculation title on a published Datasource