I think you did the right thing with your potting technique. You could also try leaving them in situ and piling soil around and above the dried-out part of the stem. Maybe even cut the bottom out of a flower pot to help to keep the soil in place.

I hope you can figure out the cause!

Linda

-- It is wilting because it is thirsty.

OTHER OBSERVATIONS:
the cracked surface indicate that you had mostly CLAY soil. It can stay soggy or can get dry and hard. Not good medium.
To correct that you need to add a lot of compost to it.

I would dig it out (wit a shovel, tryin not to disturbe the roots too much). Then prepare a big hole(~15" wide, 15" deep), add some compost OR potting soil, some manure, some native soil, a spoon full of granular fertilizer ...mix them all together and replant your plant . Bury it :DEEEP, all the way to the lowest green leaf. .. mulch the top, water REAL WELL.. and watch it grow.

It is not wilting because it is thirsty, in fact in the picture it had not wilted at all. the garden was thoroughly watered on Monday, I took that picture on Wednesday. None of the other plants, tomatoes and peppers, were wilted so they were not thirsty.

I do have some clay in the soil but it is not mostly clay and only the top crust was dry and cracked, moist and friable below. I add dirt and compost and fertilizer and till twice a year. this is not my first garden or my first time growing tomatoes but it is my first time to plant black krim heritage tomatoes.

Could we see a pic of the rest of the healthy garden?
Looks very healthy up top. Was it standing up on its own for a while?

Location, your zone, and a bit more info...when they were planted etc and if these were starts, recently purchased, your first gardening attempt...or at it for years in the same soil and this just happened...

I would mound up some more soil, even a bit from the surrounding area, then give some nice shady mulch. I use straw around my tomatoes.
Since you chose to not give supports, (fine to do that), but they supported themselves, and then toppled over from their fruit weight. A sudden bend can cause problems.
You could dig it up and lay it back down deeper and sideways covering most of the stem and the bend area.

A hay or straw mulch suppresses weeds but you don't seem to have a weed problem.

That could well be the answer! Perhaps the plant toppled over and the stem got crushed and then dried out. Re-planting with a cage or a stake for support could well be all that is needed!

Linda

For some reason it doesn't seem as emphasized as it was in gardening some years ago, but for summer crops I think it is vital.

The University of Arizona says it's widespread in the soil, tomatoes or no tomatoes. Let's hope they're wrong.

Your pic shocked so many people, not from the wilt, but from the soil surface. You can tell there's a whole slew of die hard mulchers here! It really does help mitigate some of the harsh conditions in warm zones.