I think you basically answered your own question. Pokemon had the Nintendo name, a huge marketing machine behind it, and then the factors that make it different from other RPGs at the time.

The first Dragon Warrior (the only one published by Nintendo in the West) did okay. It was only a disappointment relative to its massive success in Japan, which Nintendo thought they could recreate in the US (the same thing happened with the first Final Fantasy). But the different reception makes sense. Japanese players, even Famicom players, were already used to more cerebral games, like the popular Portopia Serial Murder Case, also from Yuji Horii. Meanwhile, many NES players had never played anything but action games, so they didn't know what to make of Dragon Warrior.

Mystic Quest I don't think ever had a prayer of a chance of going mainstream. Squaresoft just didn't have the name or marketing power at that point. They tried to bank on the Final Fantasy name, but that doesn't make much sense when even the mainline Final Fantasy games were still niche in the West then. The only people who would be drawn in by the Final Fantasy name would be established fans, and they obviously had no need for an entry-level RPG. Squaresoft was trying to tap into a market they didn't know how to reach.