If you can provide more information as to the circumstances surrounding their untimely end we can help out better. What did it look like when it died? When in the season did it did? What was the care protocol you followed for the plants? What has your weather been like?

Just wondering if yours was on the small side when planted?
Smaller Ko's around our area I notice take some time to get started.
But I've noticed the much bigger plants sold at local nurseries take off much better...
Losing Ko's after winter?
Were they grafted or own-root?

But like Josh mentions please give us more details...

By the way... If there grafted Ko's bury the bud union
4"-6" below the ground in your area.

If own-root Ko's I'd bury 2" deeper then it was in the container it came in...

Belows a video:

Here is a link that might be useful: Bill Radler on the knockout rose

Around here, they really aren't happy campers after true zone 5 winters. I've seen a fair number over the years morph into something totally different.

Frontenac is a better choice, and what I would recommend. Some of the Meidiland roses also do well, but I can never remember which ones. There are other things besides Knock Outs that will do much better past KO's hardiness limit.