Why does "pre-" change the meaning of "dominantly" to mean "for the most part; mainly"?

Rather than being a function of its etymology, predominantly sounds to this linguist like an analogical neologism based on the following relationships: dominate : dominant : dominantly : -- where all three refer to POWER -- : predominate : predominant : predominantly -- where all three refer to NUMBER. Of course, there is power in numbers, but not always. For example, Old English was the predominant language of England after the Norman Conquest, but because French became the dominant language, Old English was largely relexified to become Middle English, which can thus be seen as at least in part a French creole.

Predominant came into English fully formed, so you'd have to look to its French and Latin forebears for an explanation.