I would think the depth of the tuber is the primary reason. If they are to deep the soil won't be warm enough up where you live and to have mulch on top of the soil keeps the warmth of the sun on top of the ground.
I would dig the root balls up and move the tuber to about 3"-4" inches below the surface, remove the mulch until they are growing well and give them some nitrogen to get them growing. Dahlias like the soil to be at least 70 degrees in order to make roots. You can mound dirt around the stem that has been below the ground until it hardens off to the sun.
Another issue with the tubers so deep is drainage when it rains or when you water as they don't like wet feet and that will retard their growth or make them rot.

Thank you. I sort of came to the same conclusions, so it's good to hear them confirmed by someone else. The one dahlia that died had a mushy tuber, so that was a sign I did something wrong.

We also transplanted dahlias that were about 3 feet high to another bed and they all are very wilted! Is there a "right" way to transplant healthy plants and keep them healthy?

Papaya232,
How long ago did you transplant your dahlias? 3 feet high is a little large, but I've done it before and they nearly always come out of it and within a few weeks look the same as the ones I didn't transplant. Because I grow so many, I have a row of ones I can transplant in case something doesn't grow right in the main garden. My sister transplanted blooming dahlias a few years ago in the heat of summer because of her daughter's wedding.

Teresa