I'd say plastic is just about the best all-around material. Easily molded, strong, lighter than metals, flexible, durable, resistant to snapping and bendsw instead, cheap.

The others are specialty. Want a lighter chassis that is stiffer? Carbon fiber.
Want stronger turnbuckles that won't flex at all? Titanium.
Want a stronger metal for screws? Steel.

Etc.

None is perfect, and each has their good and bad points. But overall, Plastics are probably the best, and this is why a lot of companies use it, and because it is cheaper.