It depends on your definition of "so well." They did better than Xenoblade and Ni no Kuni, which were both considered great successes in the US. Even Joker 2, which "bombed" (due to no marketing at all, the lack of the "2 Professional" expansion pack meaning almost no real online play, et cetera) still outsold those two.
However, if you're a monolithic company like Square Enix, sales have to hit a certain amount just to even bother with, due to their outrageous amounts of overhead.
Unless those DQ games were breaking 500k-1m sales in the US, well, they just wouldn't have been worth doing. And they didn't even come close.
There's also a fan theory based in part on some off the cuff remarks from people in SE US that basically, rather than take the blame for some of the piss poor decisions made during localization (Seth Green, those god awful accents, et cetera), the powers that be at SE US just blamed DQ bombing on, well, Americans just not being smart enough to handle a good JRPG.