.diz files are not associated with text viewers/editors on any OS (maybe only on Linux), that's why to open it you need to explicitly select the text application — inconvenience out of thin air compared to .txt extension.
The .diz and .nfo files were made for directories/folders, as in FTP. You open the folder, you don't know what is this (the directory name doesn't tell you much), you download the .diz file and see what does this program do and what is it for.
Nowadays you have websites/readme in the repos for that. And even if you don't know what is that file, you can download it in seconds/minutes, compared to the speeds of the 90s.