No doubt about it, the use of raised bed gardens is probably the best there is for all home crops, root crops in particular.
Maybe they type of Turnips you planted, except for the purple tops, had a bad year because of some kind of blight or other disease that attacked to plant in some form. Not all Turnips are alike when it comes to genetics.
I learned an hard lesson two years ago with tomato plants. The type I bought were not immune to blight. And that happened during a year when two seperate blights swept through the area killing most of the tomato plants around here.
This past year I talked to a plant nursery owner about it and he recommended buying only certain blight resistant varieties, which I did. I dont remember right now what alphabetical designation those plants had which informed the buyer about the blight resistance, but they are readily available for tomato's.
I suppose that there was something genetically different about your purple top turnips compared to the others that made the others incompatible to your area for one reason or another. Most places that sell seed or plants know whether the stock they have is immune to certain problems associated to that particular vegetable. Checking catalogs may help you find out if there are resistant varieties of Turnips.
I can guess that this is not the first time you planted Turnips, so over-fertilizing and/or using a fertilizer too high in Nitrogen should not be a factor, both of which can lead to what you describe. The answer should lie somewhere between the types planted (and what they are resistant to), Ph and fertilizing. Either of those can easily be corrected.