The post-WW2 boom is dying off. Literally. Japan has smaller land area than California (a lot of it is mountains, too) and has 125mm people. They don't need more population growth. (not that you were implying they do, just saying)
Also, in my travels, Koreans were about 1000x less welcoming to me than Japanese were. Koreans literally laughed in my face when I spoke anything in Korean to them. Japanese people would get a huge smile on their face when I spoke Japanese to them. Anecdotes and whatnot. :)
I can explain the reasons behind the different receptions you received. In Japan, caucasians of European descent are largely seen as an exotic novelty; you will be treated nicely on the surface, but you are not expected (or really allowed generally speaking) to fully integrate into Japanese society. You're just a tourist or English teacher on a temporary stay. Koreans don't have this view of caucasians (though I don't know anything about whether Korean society as a whole allows other ethnicities to integrate) also imo Korea is the equivalent of Italy in Asia
I thought only white people can be racist.
And a lot of Koreans get really excited when Westerners speak any Korean at all and will praise your Korean. What can be frustrating though is that they will often try to use English to speak to you even when you tried to use Korean, instead of using Korean as well. This can also seem rude/belittling but isn't really - often they're just excited to get to use English.
Hmm, my teacher's husband said my pronunciation was excellent. I downplayed the comment politely, and he said, "No, I'm serious. You have the best pronunciation of any of our students." I'm not really in a position to judge, though. I just copied the pronunciation/phrases/tone from men speaking in k-dramas I used when learning.
None of the Koreans I tried to talk to in SK could speak English, even the young people. Perhaps they were just too afraid to try? The stares/glares also felt very unwelcoming compared to Taiwan and Japan, too. It's the only time in my life I have actually felt unwelcome somewhere.
I live in Germany, and the Koreans I meet here (we have a decently-sized local ethnicity, a multi-story culture center (http://www.kulturkorea.org/de/) and a significant Korean expat community (http://berlinreport.com/)) are more likely to be attuned to and interested in dealing with Westerners, I guess. And any Korean acquaintances I've made online (IRC, Hello Talk, etc.) are in connection with language learning activities, so they're probably similar.
The pronunication theory is based on my own experiences learning the language though (I e.g. found distinguishing and correctly producing some of the tense consonants vs. the basic versions quite tricky initially) and from Koreans admitting that it can be really hard to understand Westerners trying to pronounce their language.
I think the above sort of hints at something though: If you're a Westerner curious about Asia, it's probably a good idea to try and make contact with natives who similarly have an appetite for relating to those foreign to them. No doubt there are Koreans not interested in dealing with Westerners; there are people like that everywhere. Doesn't mean you can't find your tribe somewhere in a 50 million people population, though. It's not like you like everyone back home either, probably. Gotta expect the same anywhere you go.