Life, it has often been observed, is not fair. If it were, lawyers wouldn't exist and then where would we all be? The particular unfairness in question here though is that Harvest Moon DS is the best-selling game in the series, by a country mile, and yet is also – by an equally inexact measurement, some will say – the worst.
Unfortunately it's the only one of the more-than-a-decade-old series that ever got any advertising so that's the one people bought, despite the fact that it was a not-at-all-disguised retread of Friends of Mineral Town on the Game Boy Advance. If you must know.
Harvest Moon: Island of Happiness though is a proper DS game, designed for the console from the ground up.