Hitler actually explains this in detail mein kampf

Germany is very much a land power, they were late to the colonial game, and geographical unsuitable for that form of conquest. basically, Germany has no future in going around conquering 3rd world countries and building a big over seas empire

since, as hitler says, the singular mission of national socialism is , never the less, the acquisition of soil "by the sword", he posits that this leaves one remaining course of action: Germany must acquire her new soil from within continental Europe. this new territory must, at all costs, be directly connected to the German heartland, and a direct extension of her boarders

as such, the primary targets of national socialism was other white peoples, because it was from them that National socialism would deliver to Germany the soil it needed to stand on its own two feet as an independent super power independent of the influence and whims of other nations