>The fact that it was Slavic land? The sparsely populated areas there?

Prior to WWI, Germany had tried expansion overseas, and found itself promptly cut off, as war commenced, by much superior navies. This expansion was explained by German politicians and thinkers as a natural consequence of population expansion; one required the other, whichever way you worked it out, so post-WWI, with overseas colonies gone, the natural place for territorial expansion would be somewhere safe from naval interference, where Germans could travel back and forth without any other country intervening. Somewhere contiguous with the existing German state would be perfect, for instance. The extermination of 'lesser' races was essential to Nazi ideology, and taking land those races formerly occupied was part and parcel of that, to the extent that it was emphasised that this was the 'natural' thing to do. In order for Germany to expand and become the sole real power in Europe, it would need vast amounts of land, resources, and an even larger population to draw on for labour. The populations already living there were 'destined' to be swept away by the unstoppable might of Germany.