My thinking on the issue centers on a) skipping the SNES, b) the juxtaposition of FFVII and DQVII, and c) the nature of the appeal of DQVIII.
As to a), I think DQV at least could have been a cult classic on SNES and could've gotten DQ's roots into the hardcore console RPG community much like SNES FF, CT, and such. I think Enix was either unable or unwilling to do the legwork to build up the DQ fandom in those formative years for JRPG fandom and thus isn't as highly regarded as having oldie cult classics. I think this harms DQ in conversation around RPGs in enthusiast circles.
As to b), I think DWVII coming late to the PSX and not having the angsty melodrama, modern setting, and spectacle of FFVII and the other big Square PSX RPGs made it run distinctly counter to what JRPG came to mean to the new, enlarged JRPG market that FFVII created. I think DQ sets itself up as not meeting expectations for a lot of people into JRPG in the west and this starts with the FFVII/DWVII divide. Moreover, given a), there is very little counter-current.
b') I think in more modern times, DQ also simply isn't the product, for which people interested in Japanese games are looking. That is, I think people interested in Japanese games are generally looking for pre-established brands and otaku content. I think DQ fails to be attractively "Japanese" to a lot of people in enthusiast circles who want other things from Japanese cultural product.
Basically, summing up b) and b'), I think DQ is a "niche" product in the west that doesn't appeal to the core audiences precisely because it is a mainstream oriented product that has missed the mainstream. So the question sort of becomes "why wasn't it able to Pokemon?" and I think there concerns of marketing, image, and infectiousness of the core conceits come into play.
The other problem with b) is DQ mainline does do fairly well with itself among similar games but doesn't live up to its pedigree. So honestly b) and b') might well be bullshit.
As to c), I think DQVIII resonated as "they translated the SNES/PSX JRPG onto the PS2--it is charming but also a throwback, that is DQ is stuck in the past and sometimes that is refreshing but DQ also is uninventive, boring, and being stuck in the past is also somewhat off-putting."
After DQVIII, DQ also has a spotty record in the west: Jumping from PS2 to DS in the west isn't the same as doing such in Japan. DS/3DS DQ remakes sold poorly in the west and the 3DS ones came over "reluctantly." DQX skipped the west. DQ spinoffs sold poorly on all systems. And now DQXI is essentially a new beginning for the series other than trying to call back on VIII nostalgia.
So I think there is an element of mismanagement of the franchise wrt the west even after DQVIII--the platform jumps aren't ideal for the western market, DQX skipping the west isn't ideal for the western market, and, going with what Linkura said, marketing was poor in the west.
I also think there is an element of western appetite not being there for DQ. I think this is, in part, Enix's fault, going with a), but also Enix is a victim of not having their ducks lined up when FFVII shaped RPG fans, going with b), and never managed to become "quirky, cute, stylish, sexy Japanese" cultural product, going with b').
