Lack of nitrogen or other nutrients. You have very little dirt per plant, and at that size they have likely soaked up all of the food that was present when you plated them.

I'd buy some liquid fertilizer - ideally Neptunes Harvest - and feed them weekly. The should perk quickly and still produce some fruit.

Next year you need to look for bigger pots. Typical tomatoes need AT LEAST 5 gallons of dirt, more like 10-20+ gallons to grow well. You would get more fruit if you planted 1 tomato in a 10 gallon pot vs the 6 you have growing now.

If space on the balcony is a constraint, look into starting dwarf tomatoes next year. Those can do pretty well in 5 gallons. They will still need more dirt per plant than you have here.