It's because the two words do not mean the same thing! Disinformation implies dishonesty, misinformation does not.
That's interesting, my New Oxford American Dictionary (3rd edition) treats "misinformation" thusly: Though the fact that the end of the Cold War has not been kind to disinformation's prevalence (see Google Ngram, above) suggests that the distinction you raise between dishonesty and honesty is a salient one.false or inaccurate information, especially that which is deliberately intended to deceive
Perhaps I should have said, "Disinformation specifies dishonesty, while misinformation merely implies it" :-) After all, the same dictionary treats "disinformation" thusly:false information which is intended to mislead, especially propaganda issued by a government organization to a rival power or the media