Exactly why does Japan and Korea have frequencies not supported in the U.S.?

i know that atleast their cells run off of 2100mhz only, and that frequency is also the only frequency they allow. Why don't they allow 900/1800/850/1900 frequencies as well when most other nations do? Also is this america's problem for technological advancement?

also im merely asking this because many large countries support all kinds of frequencies, including 2100, but japan and korea stick to 2100 only, and the u.s. does support 2100 hdspa; but take the average american quad band phone and it still won't work there. very few people are able to get reception with that

Because they are different countries run by different governments.
In Korea.
SKT offers service in WCDMA and CDMA with EVDO for data. WCDMA is 2110 MHz to 2170 MHz and they split the range with KTF. The CDMA EVDO is 869 MHz to 894 MHz and SKT pretty much owns all of the bandwidth.
KTF has pcs phones in the 1800 MHz range. These are use CDMA modulation as well and support the EVDO upgrade.
KTF also has WCDMA in the 2100 MHz range.
South Korea uses the same cellular band as the US the real difference is the modulation used.

Not sure about Korea, but the short answer to your question regarding the US and Japan is that the US uses 2G technologies (GSM/EDGE) and Japan has a 3G network ( W-CDMA, 2100MHz). The problem is not only Mhz. The entire structure of the networks is different. Even if Japan were to allow backwards compatibility with lower frequencies (why bother?), phones made to work with EDGE networks wouldn't work.
BTW, that is why the iPhone does not work in Japan.

I, too, believe that the US is the center of the world, but why do you think Japan and Korea should mold themselves to a US standard?
Korea, especially, is one of the world leaders. It is the most wired nation on the planet with more high speed internet access than anywhere else.
The question should really be, Why hasn't the US kept up with Korea and Japan?

Perhaps the reason the reception in Korea is so good is because they have remained simple instead of trying to cater to every whim.