Sharepoint - Why are fields in Datasheet View read only?

From my experience, this is not possible through the SharePoint Dataview Sheet View. This is a new feature in 2010 and although super delicious and powerful, it still has a lot of issues that need to be resolved before it will be as powerful as it can be - in 2013, Microsoft has made improvements as per this article from MSDN

To answer your question - in no state are metadata columns editable in datasheet view. One of our clients had the same issue and they ended up going with ShareGate's migration tools. They have a metadata editing tool that runs for about $395

There is also a free tool on CodePlex (which I have not tried) that could be worth a try - from the comments, a lot of people found it helpful: WinForm SharePoint list item editing tool

I stumbled across a way to fix this today.

Under "Versioning Settings" there is a setting called "Require documents to be checked out before they can be edited"

By default, in all my libraries, I have this set to "Yes"

I discovered if I change that to "No" my datasheet views become read/write.

Odd? Very. But it does seem to work.

Incidentially, I flip that to "No" only when I'm going to be making mutliple changes to a document property in datasheet view. Once I'm done, I flip it back to "Yes"

I changed the text type from 'Enhanced Rich Text' to 'Rich text' in the column setting(through List setting) from SharePoint and it worked for me.