Why are text files not supported by Quicklook?
Normally, text files are supported by QuickLook. If you have one that isn't, chances are that there is something "odd" about the file, at least from the Finder's point of view.

Try opening the file in TextEdit and then choose "Format" and then "Make Rich Text" as shown here:

http://images.luo.ma/TextEdit-Format-Make-Rich-Text.jpg


Type a space (or modify the document in some other minor way).
go back to the "Format" menu and choose "Make Plain Text" again. 
Save the file. 
Check to see if QuickLook is able to show the file.