In the euro area and other major advanced economies, productivity and population growth, which are also the drivers of potential GDP growth, have been declining for decades. The intersection point of the desired demand for “loanable funds” with the level of savings constitutes the interest rate.
In addition to these secular forces, more cyclical factors linked to the financial crisis have been a 3/7  BIS central bankers' speeches  drag on real interest rates. In particular, the euro area has faced a massive debt overhang in the public and private sector, resulting in a severe “balance sheet recession” and a tightening of credit conditions.