My first thought was "soil disease" when I heard that you had good crops up until a certain year, then plants started dieing young. One possibility was roots dieing young, then the plant drying out and dieing.
If you cleaned pots and garden tools really well before starting plants in pots of potting mix, that SHOULD rule out soil diseases. Possibly mixing potting mix in a wheelbarrow and with a shovel or hoe that had soil on it could spread a root disease.
Was there any source of contamination between old tomato beds and new pots? I'm guessing that soap and water might not be enough. Chlorine bleach or hydrogen peroxide might be needed IF some hardy soil microbe is the villain.
Can soil pests like insects climb into pots?
Can microbe spores in soil be blown with dust into pots?
Is the climate suddenly so much hotter and drier that there's no way to water them enough? It sounds as if you are used to HUGE plants that need root systems like the NYC subway tunnels.
Do you irrigate with town water, and the town water supply was recently "improved"?
You might try another crop in pots, or in some spot in your yard as distant as possible from any bed where you grew them for years. Maybe the tomatoes in the ground died for one reason, and the pot tomatoes died for a different reason.
It doesn't sound like an 18" plant could already be pot-bound or root-limited, but I imagine that any plant that could produce 57 kilos in a year would need a 10-20 gallon pot, and/or being watered twice every day and fertilized weekly.
I'm just guessing, really.
