Why Do Certain Morgan Dollar Dates Tend To Be Toned More?

I like Morgan dollars, and I prefer what I consider to be nicely toned dollars. Why is it that certain dates and mint marks tend to be more readily available toned than others? Some dates and mints seem not to be available at all. I have two theories that may or may not be valid. One date and mint mark that tends to be more toned than others is 1885-O. Perhaps the New Orleans mint received bags that were more conducive to toning coins that year. Another theory is that the 1885-O is relatively inexpensive and nicely struck than others and folks who artificially tone pick that date because the downside is not as great should they get a "bad" toning result. Am I missing something obvious here?