I've experienced this too, both here and in OH, even if I put them in the ground the day I buy them. I think it's a combination of things... They get very thirsty yet need great drainage. They are usually extremely rootbound in peat and vermiculite, which are substances that create inherently weak root balls, IMO/E. The fall-bought blooming plants have been artificially manipulated to be blooming at a later time than nature normally dictates, and are not all hardy perennials where they are being sold. Although fall is a great time to move/plant perennials, mums seem to be an exception, or they may just be freaked-out by the manipulation. The ones that do survive are always short-lived both here and when I lived in OH. The ones that have survived for me for at least 1 winter were almost always smaller plants from 4- or 6-packs.

I would say that if you are buying them as perennials, your money might be better spent on something else. If you feel like you've got your moneys' worth with this years' flowers and perennial-ness would just be a bonus, go for it!

Why don't they ever sell these in the spring so one can grow a HUGE, more healthy plant, even if it's still going to die over winter?

Like purpleinopp, I think that the chrysanthemums you buy in full flower in the Autumn have been carefully forced and nurtured to looks fantastic on display but are not necessarily tough enough to take the transition to the open garden. For hardy perennial chrysanthemums it is probably better to buy them from specialist growers who sell them as garden plants, not temporary seasonal decoration.

Planting in the spring is the best advice.

In MN mums are often sold as mums or hardy garden mums in the fall. Those that are not labeled as hardy garden mums will usually not survive the winter where most hardy garden mums will survive.

If it is late in the fall I often place perennials, including mums, still in the pot, in the ground and cover with mulch to overwinter. The next spring I lift them out of the pot and plant in the same place. I think this helps as the roots are not disturbed so don't have to worry if they will settle in before the ground freezes. Also, I remove as much of the planting mix, mostly peat, from the
mum roots as I can before planting in garden. That mix dries out faster than garden soil.

Do you know about the mammoth mums (first sold as My Favorite, years ago) developed by U of MN? They are very hardy and grow quite wide (unless you keep cutting off a piece to share) and do not need pinching to keep them compact.

If you can find smaller plants in spring, plant them then. Most fall mums are mass produced, and rather tender with the amount of water and fertiliser applied, and then sold in full bloom. If you plant them in spring, they have good conditions to grow and set roots before they start expending their energy in flower production.

If you can do it, knock or lift a mum you are thinking about purchasing out of the pot (the soil and root ball should stay intact and not fall apart) and look at the roots. They should be white or at least not dark and hairy and healthy looking. If they're not, that's an indicator they're probably not going to last the winter. Then please put them in a place where they'll get good winter drainage. Most mums die from rot. Mulch in winter, but don't be in too much of a hurry to remove them when the first warm days of spring come along. I killing freeze can finish off the crowns.

Yes, it's true some varieties are a lot hardier than others, and there isn't any magic formula other than experience to differentiate which ones. Both may be sold as 'hardy' mums. I have some stands of mums I know have been in ground at least 30 years and still cranking. Others not so fortunate. Their price is so low on the market now, if you buy them even in fall just starting to throw bud, they're usually worth the price, even if they end up only blooming the season.

Here are some suggestions for more reliably hardy mums (which you probably won't find at most garden centers in fall).

{{gwi:23034}} is an excellent performer for me.

Ordinary potted mums have a high failure rate when it comes to winter survival, especially when planted in fall in colder regions and when there is no basal growth at the time the plant is purchased.