There are so many possible reasons your caterpillars didn't survive, I don't know where to begin. Were their food plants recently bought? They could have had insecticides on or IN them, like systemics that the caterpillars would ingest with the leaves. You'd think people wouldn't use poison on a known host plant for butterflies, but some nurseries do just that, so the leaves will be pretty and the plants will sell better.
They could have had a disease, like OE or any of the other things that beset monarchs. The soapy water might have hurt them, I don't know, never having done that.
Maybe some of our forum members who are more experienced with monarch caterpillars than I am will be able to help you.

Sherry

I did purchased plants from local nursery. I just purchased a few butterfly weed from Lowes. I haven't found any new cats in a few days.

I would be careful with the Milkweed from Lowes. There is no guarantee that it hasn't been sprayed. I never buy it from a big box store unless I see BIG caterpillars on it.

At one store(different town) the plants were covered in eggs but not in my local store. Not sure what's going on.

Don't worry about the aphids as they don't bother the caterpillars. Maybe the soapy mixture was the problem. Do you have a different container? If so move them there.

I finally got the sanitize solution from live monarch. I got new containers and now waiting for new eggs to hatch. Will see what happens on this next munch of cats.