Because for my whole life, I have been unsuccessful in getting more than a few tomatoes from each heirloom plant. I used to think it was the soil, but now, in TX summer heat, I'm guessing its like trying to grow everbearing strawberries. By the time they would start bearing fruit, its too hot. So, I thought I would give determinates a try. My tomatoes that I set out in march are now 15' tall, and still growing. Not one additional fruit, yet. The second session tomatoes set out in June, have a few fruits, ( as usual), but so far though still growing, they're not setting much fruit. I do have one determinate I bought in July, I think. It has no fruit either, but it is smaller. I fear I bought it after the fruiting season for determinates, so it may be too late.