Double wall= twice the weight, + 14 psi on the OUTSIDE of the tank, which is weaker, structurally, than pressure on the inside. Ever crush a beer can? It takes 50+ psi to make one explode.
Heating the tank raises the temperature of the wall, causing the LOX and liquid hydrogen to boil. Boiling = excess pressure = BOOM!
Thinking outside the box can bite you on the fanny...
An outer wall need not be pressure tight -- all it need be is enough to keep the foam inside from shedding away during launch.