Without being there to dig them up and look at their roots (which is what I'd do), I'd say that because those tiny zinnias were forced to bloom so prematurely in a greenhouse, when you disturbed their tiny roots to transplant them, they couldn't support the flowers and grow new roots at the same time. Geez, the poor little things barely have enough leaves to support photosynthesis. One other cause could be that you didn't press the soil firmly enough around those little roots and they died from air exposure — I've seen that happen to tiny 6-pack plants before.
A tip for buying the best bedding plants: don't buy the ones that are blooming. You want them to establish good roots first. Also hold off fertilizing them until they are established and looking healthy.
If they haven't already expired, I'd suggest clipping off the blooms, making sure the soil is firmly pressed around the roots and waiting. But I'm not too optimistic.
