Because grapes native to eastern America are very different to those found in Europe; varieties like the Concord, Delaware, and Niagara contain a lot of
https://en.wikipedia.org/wiki/Methyl_anthranilate which the old world would consider floral (and is used a lot in perfumes) while most Americans think it tastes like grape.
This is also different to the Californian seedless grape, which is closer to those found in Europe.
