Because the alumilite is not actually bonded to the aluminum, and the aluminum is not all 1-piece design and cast in a single part.
You can glue the parts back on that snapped off, but that won't fix the underlying issue ... the aluminum honeycomb is dividing the alumilite into discrete blocks that have no support other than the aluminum, which isn't a solid piece.
You can try reinforcing the aluminum by cutting it thinner, and sandwiching 2 layers with aluminum with a single solid layer of alumilite in between... or you could go with a more solid piece of aluminum to hold the design together... The sandwiching method should prove more economical and easy to do. Keep the 2 layers of aluminum honeycomb from touching by adding spacer blocks in between while casting.