DQ8 already has an english localization. DQ7 was remade and would've needed a new one. That's a huge difference and it's the main one that tends to be the deciding factor, especially with such a text-heavy game.
To release this in the west, all they have to do is go back to their english script and just copy it into the new game and then probably get at least half a million sales out of it. They'd be absolutely insane not to do it! Whereas with DQ7, it's a huge game with lots of translation to do. It's also a lesser known game in the west. That's a much bigger gamble for them. DQ8 is no gamble at all, it's guaranteed profit.
English translation for the new DQ7 out as well. Many times companies like this pay to use fan translations. Xseed almost entirely uses them for Falcom games. DQ8 also sold rather poorly..only half a mill in the US and that was only because it included a demo of FF12.DQ7 didn't sell great either but it was released late. That said they sell on DS but then again this is the same company who hyped people up at a conference to announce a port of a port of a port and so far yet to release it over a year later.