Because they used incorrect materials and insufficent engineering.
I admire the project, but you seem to be the only person who points out the MAJOR flaw, overriding flaw, on both of these beasts...massively overweight, and underengineered, and an accident waiting to happen...whatever form that accident took.
Two different forms so far.
The fact that the second one came out a HUNDRED pounds lighter, what does that say?