Because it's programmed for US zip codes.

So, you can't just enter any zip code in there. There would be clashes since zip codes aren't unique across countries.

Just to be a pedant, that's not what was stated: "As a workaround, you can use your zip code instead.".

Your assumption is that zip code == "U.S zip code" which is a fair assumption if the site only supports the U.S. Other comments on the thread shows the site does support locations outside of the U.S. It is therefore also a fair assumption that it _might_ support similar functionality outside the U.S., kuschku simply tested said assumption and stated that it was incorrect so that other readers were aware.

For that use case there's the ISO standard for ZIP codes, as used in paper mail:

Short Country prefix (same as on international car ID plates), then a minus, then the ZIP code.

So, for Germany, D-24107 would be one such code.

Yet, it doesn't handle that either.