Why couldn't this series have been bigger over here?
I know people tend to say it's because advertising, but I don't think that's the only reason why.
The first thing that stands out is that the only Dragon Quest game we got for the Playstation 1 was a 2.5D RPG, it came out at the end of the Playstation's life cycle and at that point people were already impressed by Final Fantasy 7, 8, and 9 people wanted more more visually impressive RPGs and they were looking forward to PS2 RPGs.
Which brings me to the next reason, there just wasn't enough DQ games being localizedd or developed to keep the franchise relevant in the west. During the Golden Era of Japanese RPGs (which I count as fourth gen to sixth gen) we only got 2 new titles mainline titles, 7 and 8. Even if 5 & 6 were localized (Enix NA got denied to localize 5 and went bankrupt during localizing 6 iirc), it was really the fifth and sixth generations of consoles that boosted the franchises in the west, FF and MegaTen are both examples.
I could be wrong, those are just my assumptions. I was only 2 years old when FF7 came to the west and didn't start taking the gaming industry seriously until midway to the 7th gen life cycle.