Because the roots are bound in those small buckets.

Plant your next ones in 20+ gallon Smart Pots with good potting mix and you have the best of both worlds.

You say your ground based ones have few blossoms? I had this problem a few years ago; incredible plants, healthy, robust, dark green, and BIG! But few blossoms and few tomatoes. A master gardener laughed and asked me how much I had fertilized and with what. He told me to get a soil testing kit, which I did. My NPK were all very, very high. The culprit was too much nitrogen. The tomatoes put all their effort into growing, not producing fruit. Even if using the same fertilizer product in your garden and your pots, the N could be higher in your garden soil than the pots because in your pots the N will leach out faster. Oh...and I have had tomatoes (not dwarfs or cherry) produce very well in 2 gal pots on my deck.