1. In the west, JRPGs simply weren't that huge. I suppose the NES Dragon Warrior games sold well enough since they kept putting them out, but there wasn't the JRPG boom we saw in the 16 bit and especially 32/64 bit era.
2. Generic fantasy box art and title. I don't think Dragon Quest is that great of a name... but it's still better than Dragon Warrior. You might say that changing the boxart was simply localization as anime wasn't popular in those days, but keep in mind there were a TON of NES games with fantasy art you'd likely see on the side of a van. Nothing made it stand out if you were just looking through NES games in a store.
3. DQ III and IV were released in 1992 in the US. Yes, they released both of them in the same year. After the SNES and Genesis came out. Sure, the NES was still massively popular at this time and new games were coming out, but these games were ugly compared to others that utilized Super FX and Blast Processing™. DQIII is especially regarded as the first great DQ game and a bonafide classic of the 8bit era, but it was buried.
3. No DQ games for the SNES in the states. JRPGs started to build momentum at this time, but DQ wasn't represented at all.
4. Enter the 32/64 bit era. FFVII has reshaped gaming and the JRPG genre has exploded. Where's Dragon Quest? Nowhere. The game doesn't even release until the year 2000 in Japan, at the end of the PS1's lifespan. It his the US in 2001. One week after, I kid you not, GTA III. Ouch. I do remember this game getting a bit of hype and marketing push however. At this point, everyone was aware that DQ was crazy popular in Japan, and since JRPGs were big, any new one was on a lot of people's radar. Not to mention it had VII, like FFVII. Considering it's coming out years after FFVII, I beat it blows it out of the water graphica-
Oh... I'm gonna play my ballin' PS2 instead. I don't know what the reception was in Japan, but DQVII got some pretty mixed reviews when it released in the US for being an old ass slow ass feelin' game.
5. DQVIII and DQIX did well in the states. DQVIII was like a reboot for the series in the west, and the impressive graphics and rich content captured a lot of new fans and it scored well across the board. DQIX was on the DS, that thing everyone owns. Though this may have had a negative effect in the west as console DQ fans were left in the dust.
6. DQX doesn't come to the states. Can't blame 'em, it's hard to release an MMO. But when DQXI hits the US next year, there will be a EIGHT YEAR GAP between the last two mainline DQ games in the west and a TWELVE YEAR GAP between console Dragon Quest games. That's a lot of time...
7. Since DQ games have several bibles worth of text, it takes a long time to translate. With many franchises, even Final Fantasy and Pokemon, becoming worldwide releases, DQ not being one kind of kills the launch hype.
I wrote way too much about this subject, but it's a mix of not getting a strong start in the west and some unfortunate timing.