Why is the ten year interest rate lower for Spain/UK bonds than for US? Take a look at the government bonds interest rates, https://www.marketwatch.com/.

For the US is 2.9, and for Spain and UK are 1.5 and 1.1 respectively. Spain is an economy with high unemployment, a high deficit and debt and low growth. The UK is suffering through Brexit, massive uncertainty on its future. Meanwhile, the US solidly grows. 

Why would investors charge more to the US, perhaps the most powerful country in the world, than to other countries?
Inflation forecast is higher in the US than the rest of countries. US here: https://www.statista.com/statistics/244983/projected-inflation-rate-in-the-united-states/ 

Spain here: https://www.statista.com/statistics/271077/inflation-rate-in-spain/

Japan here: https://www.statista.com/statistics/270095/inflation-rate-in-japan/

UK here: https://www.statista.com/statistics/270384/inflation-rate-in-the-united-kingdom/

Because inflation is higher, central banks will raise interest rate higher. Long term rates are higher in consequence.