The series has failed to create a sense of identity in the West and their releases missed the interests of its potential fanbase by releasing the mainline games and remakes on handheld systems and the spin offs on consoles.
While most of the fans buy the games as they are released: be it handheld or console, we do it because we already know about the series. It's very hard to introduce a newcomer to a RPG released in old hardware, and much harder if we are talking about an adaptation/remake of an older game from a series that is known for their traditional style of gameplay. While the latest releases have matched the interests of their most important market: Japan, they failed to meet the expectations from their potential western fanbase. Newcomers are not drawn into spin offs like Heroes, since they can not enjoy the fanservice component these games have and releases like Builders, which actually cater to a genre that is pretty popular overseas, are brought down because people just don't know about the brand. In past years, the series started to gain mainstream appeal thanks to DQVIII, which was pretty similar to the most successful JRPG released during the PS2 era and would cater to an audience that demanded more of those games. DQIX also found its way into new players thanks to being released on the most popular system at the time and counting on good word of mouth. However, time has shown that the demand is simply not there for remakes and re-releases, and since the series got to a stop gap these last years before releasing the next mainline entry, the interest in the series has lowered increasingly.
That is why Dragon Quest XI is an important release (and I would say, crucial. If DQXI bombs we can kiss the series goodbye in here) in the West. It marks an entry point for a new mainline game released in the leading console in western market and in a brand new system that seems to be rising in success. Not only DQXI builds on VIII, one of the most known entries in here, but also feels like the most up-to-date game in the series and the easiest way to ease a newcomer into the series. The releases of DQVII and VIII on 3DS failed to bring a Persona 4 Golden moment to the series overseas, in which new players finally learned about the series and started to anticipate future releases. They could have stood a better chance by being released in another platform, maybe, but it's also true that they were not promoted neither advertised enough for them to catch mainstream attention.
If S-E tries again with DQXI and the western gamers still don't care, then I would understand if they just simply gave up for the time being. I don't know what excuse would there be if the game underperformed.