Why are imported Italian canned tomatoes more tender than the same American product?
Unfortunately, it's a known problem. Much of the produce in the US has been bred for profit: size, color, amount produced per acre, and ability to survive handling (mechanized pickers, shipping long distances, etc).
It's that last one that tends to be the problem with tenderness -- a tomato that's tender when ripe will get squished when it's being picked in the field and transported to the canning facility: https://www.youtube.com/watch?v=Rhu21aNUlxQ
(summary of the video : vines are cut and stripped, then tomatoes are flung into a large container where tons are collected at once, then they're washed, sorted, stripped of their skins, and possibly diced or crushed, sorted again, then canned and sterilized)
Now, canned tomatoes are better than the ones you get in the grocery store -- they don't have to be shipped cross-country (or even from South America), they only have to make it from the fields to the canning plant ... which are typically built in areas where there's lots of tomato production. But they're still shipped in large containers, so the bottom ones need to be able to survive the weight of the ones above them, and they need to be sturdy enough to survive the dicing process.
And so there's one other trick that North American companies do -- they'll often add calcium chloride to the tomatoes to keep them firm. But this also makes it more difficult for them to break down when cooking.
Cook's Illustrated did a review of canned diced tomatoes years ago, and they mention that different brands use different varieties of tomatoes ... but you'd have to get a subscription to see what they preferred. They also mention different levels of citric acid used, and acidity can slow the breakdown of some vegetables (onion, potatoes, etc.).
Your best bet is to get tomatoes from a local, small farm but then you'll have to peel them yourself. If you're making sauce, you can ask them for 'seconds' which might not be as pretty or blemish-free like the ones they put out at farmer's markets.
If it's not the right season and you need to used canned, check out the labels and look for one that doesn't mention calcium chloride.
San Marzano Tomatoes are grown in Italy. On one particular piece of real estate and on one side of the mountain which give you the best quality in all of Italy and the World. Tomatoes that grow close to this real estate have much of same quality but can not be called San Marzano. Remember, the tomato is a fruit. Italy is also responsible for incredible wines made from it's amazing grapes grown in unique soil in which volcanic ash of Italy's past may play a role in its produce quality. Sometimes tenderness depends on when a tomato is picked or the way they are processed when it comes to American Brands.