I believe it's because it's one of the words that has a great many variations in the pronunciation. Compare an Indian saying Bad to an American, to an Irish person to a South African.

If I am reading the article correctly, they are predicting the death of "bad" not because different English-speakers pronounce it differently, but because there are few other languages using a word with the same meaning derived from the same root:

For example, "dirty" is a rapidly changing word; currently there are 46 different ways of saying it in the Indo-European languages, all words that are unrelated to each other.