Soil composition, fertilizers, vegetable strains, and weather conditions all impact taste. Tomatoes from my garden made from heirloom seeds taste significantly better than best boy, beefsteak, or even tomatoes on the vine. I've had tomatoes from Italy, in Romania if all places, and can concur that even the produce exported tastes worlds better than what you can buy in the average US supermarket. I'd wager that the type of produce grown in the US is the culprit as it tends to be more important to have shelf stable and attractive produce than tasty produce. Stuff from my garden or the farmer's market is comparable to produce I've had in Europe.
You can find heirloom veggies and less industrialized produce in American markets — but part of the reason they're not quite as popular is that they don't ship as well or have as long a season. Fuerte avocados, for instance, have a season of about six weeks. They're delicious but towards the end of the season they're not that great.
Of course, living in the Bay Area I'm spoiled by the selection. It really is better than a lot of other areas. In contrast, Portland, in particular, stood out as having consistently mediocre produce available (and New York was surprisingly good even in the middle of winter).
New York and a good chunk of the Northeast get a lot of produce from Canada year round. New York is also a major port city with fresh food coming in daily. I can't find any heirloom anything in my local market, not a big enough draw. The farmer's markets are great though fresh from field to table and usually great prices from great people. In the Bay area you actually have a really nice pipeline to farmers which probably explains the great produce. Portland seems like it might have a rain problem with would limit produce to tolerant it greenhouse crops, especially tomatoes which do poorly in tons of rain. Even lettuce needs to dry out lest it rot.
