Because melamine is clean, effective, easy and cheap.

Melamine is really very easy to use and scratch, stain, and water resistant. I have minimal carpetry experience (shop class in the 8th grade) and I was able to work with melamine well enough to make a very nice 4 tub rack.

If it came to caging, however, I would go with plywood + a combonation of tileboard, plexiglass, and sealant.

Bang for your buck though, melamine is the material of choice.