Why do other border walls work when ours does not?

Greece just built a wall on the border with Turkey, ostensibly to keep our Afghans, but will also work to keep out Syrian refugees and other refugees as well. Why does it work, when the US border wall with Mexico does not work?

There are a couple of reasons that these two walls (and a third wall in Hungary) diverge in acceptance.

• Shear length of the walls makes construction more difficult. This is magnified by the topography of the land (the US / Mexico border is very very rugged, the others are not. 
• The Greek wall is approximately 25 kilometers (or about 15 miles) long. And runs the length of the Greek / Turkish border.  It is 1.3% as long as the US Wall.
• The Hungarian wall is approximately 523 kilometers (or about 325 miles) long. And runs the length of the Hungary / Serbia border. It is 16% as long as the US wall.
• The USA wall is approximately 1044 Kilometers (or about 649 miles) long. The Rio Grande, not much of a barrier but one none the less, is 1997 Kilometers (about 1,241 miles). There was less than 25 miles of wall in 2005.
• This makes the US border harder to hide being a wall, and MUCH more expensive
• Other walls discourage movement from only a portion of the entire border and only on the border of non-allied neighbors.
• The Greek Border wall faces only Turkey, less than 10% of it's foreign borders The Greek borders with Bulgaria (another EU member) is not fenced. The borders with Albania and North Macedonia, poor neighbors trying to get in the EU - so not migrating, are not fenced.
• The Hungarian Border wall faces only Serbia, about 25% of it's borders. The rest of Hungary borders EU partners (and a small slice of Ukraine).
• The US Border wall covers our entire southern border. And the other side is an both an ally and massive economic partner of the United States, Mexico. So that border MUST be porous at some level to allow massive trade.
• The US is NOT a single race, so it is harder to define who is or is not an American.
• Other countries that allow in foreign workers, do not grant citizenship to other nationalities. Even to foreigner born in the country - for example even 3rd generation Turks in Germany cannot be German citizens (same is true for many countries, like Japan).
• Almost everyone who is a Hungarian citizen is a Hungarian by ancestry. And Roma or refugees cannot be citizens.
• Almost everyone who is a Greek citizen is Greek by ancestry. And Turks or Syrians definitely cannot be citizens.
• Americans, except native Americans on reservations, is a mix of ancestries. Therefore it is much easier to build a wall and play to nationalism and xenophobia in single race countries.  In America it is much harder to define "non-Americans", because it is possible for everyone to be an American. So once past the wall or immigration control at our airports, it is very hard to discern who is legal and who is. here illegally.
• Finally, the border wall has become political.
• Before President Trump, the border wall was controversial, but not by political party. It wasn't until the 2000s and the advent of Fox News and President Trump that Republicans politicized the wall. Before that, both Republicans and Democrats were in favor of building the wall AND providing a path to citizenship. But once you blame Democrats for the problem, it is suddenly a mark of political affiliation, not a solution.