But, regardless, the top 100 includes wines from Australia, Chile, and Germany. Spain produces more wine than these countries combined. Is there some historical reason why Spain is not represented?
Yes, there is.
Until the last ten years or so, Spanish wines, with the exception of fortified wines, and that primarily Sherry, and that primarily very bad Sherry, some of it "cooking Sherry (i.e., fortified with salt) didn't exist in this country.
And that was in large part because, until the last several years, when the Spanish got on a solid economic base to be able to afford upgrading their facilities (stainless steel tanks, improved fermentation techniques) people in this country would not drink Spanish wines other than Sherry. Whites were seen as primarily old, tired, oxidized, flat and essentially undrinkable. Reds were a bit better, but little outside of Rioja ever made it into the country, and even most of those didn't suit the American palate.
Then it took some enlightened importers (and one hell of a lot of expensive marketing support) to break into the US market, first in high end or hip restaurants, then eventually into fine wine shops, and then onto the shelfs of grocery stores and such.
Back then, there was no Albarino, no Rueda, no Catalunya. Those are all very, very recent phenomenons.
The other areas you mention, Steve, didn't have those obstacles to overcome.