Most likely, your mums just kind of "faded away." Mums need to be divided every couple of years or so to stay vigorous. Otherwise, they start dying out in the centers, and as the years go by, they fade away more and more. Another possibility is that the crowns of the plants didn't have enough protection and got killed during the winter. I can only guess that the purple ones stuck around not so much because of the color but because whichever variety they were were a little hardier than the other ones. As far as recommendations for strong mums, I like the 'My Favorite' mums, which you can get in just about any color. My personal favorite is the 'Clara Curtis' mum, but if you don't like pink, then that's not the one for you. The 'My Favorite' mums don't need any pinching or deadheading, so that's always nice. I can't remember whether I saw them at Lowe's or Home Depot. One or the other :-)

Good luck!