As others mentioned, different companies. More importantly, for non-US folks, as far as taste goes Dove is a bit better quality than Hershey garbage and even other Mars brands (M&M, Snickers) as it's their "premium" chocolate, but it's still pretty waxy and low quality overall. Given the price, that shouldn't be surprising, though, you can get a bag of the stuff for just a few dollars.

American chocolates are generally sweeter, but not always low quality (nor vomitty, like Hershey's); it can be quite good, especially if eaten slowly and with coffee or something. We sometimes bring See's chocolates for Japanese relatives for a bit of a novelty, and funny enough a few of them actually like it more but I assume that might be an Okinawan thing.

Anyway, it's like comparing a fast food burger to a restaurant burger, they're completely different animals, figuratively speaking (and sometimes, literally, hehe).