The Us Consumer, and the US government have a long way to deleverage. In a deleveraging economy income is used to pay down debt,not consume. Consumption is the largest part of aggregate demand, therefore this behavior will be a significant drag on GDP.With a slow growing economy inflation is not a threat, therefore real and nominal rates stay low.
On demand side, Treasury buyers are lining up: China, Japan, Oil Rich countries, big banks, etc.
Also, Fed has no choice but keep interest rates low. Higher rates will bankrupt Uncle Sam and Americans soaked in debt; everybody would be dead. So, Fed will sacrify $ to keep rates low. A 1% downward shift of 20+ year Treasury yield would bring down TBT to $30.