The class was originally born out of a notion that FWs at non-spinner events are boring (not my thoughts) and that robots armoured up to eye balls knocking into eachother gets dull (yes my thoughts).

I later noticed that, although very inspiring and impressive, seeing some of the other robots that have a fortune spent on them and being built by some very experienced people, to be quite daunting and off putting.
By having a limit on materials, in this case just the one which is readily available and cheap, you can tackle this problem.

This evolved into the class we have now including elements such as, easy/cheap build and fix, encourage people into the sport, easy tool use, level playing field and actually having some destruction.

Indeed HPDE encompasses some of these elements, but it's still much more expensive than MDF and doesn't break as easily.