There's a lot of things to keep in mind, some were mentioned.
1.) VII was coming off of only Game Boy titles in the States. IV was released way in the early 90s and only sold around 80,000 copies. It had no real momentum of its own behind it to help push it like some bigger RPGs do.
2.) The game looked pretty much awful outside of the awesome battle animations. The FMV was atrocious and the main characters were terrible, two frame animated sprites that barely resembled their original art.
3.) It came out when the PSX was on its last legs and received a relatively small run of print ads.
4.) VIII is the first great looking DQ title (although I think 6 looked good for its time). It's also the first time that Akira Toriyama's designs really have been obvious in the main characters. I think this alone will help the game out, especially considering how DBZ like a lot of the characters are. That anime is still surprisingly popular for whatever reason.
5.) The game looks great graphically otherwise, which will definitely help.
6.) This is probably the most important -- They're not just Enix anymore. They're Square Enix. While this might not matter much in Japan, it matters a LOT here. Square USA has more pull and marketing muscle than Enix of America could have ever had in its wildest dreams.
Between that, fans of the series and people who will likely be interested just because of the Square Enix branding... I'm positive it'll be the best selling DQ game in US history. I expect it to do pretty damn well.
