Why are wetsuits predominantly black?

Answer

It's easiest/best to make the base material of a wetsuit - neoprene - black.

Most colored wetsuits get their color from being coated by a thin textile layer. Apart from offering more design options, this makes the wetsuit more resistant to wear, but also make the material stiffer.

Answer

No. Modern wet suits come in various colors depending on the manufacturer. Custom suits can be made in any color.

However, it is true that most generic wetsuits are manufactured in black or in dark blue. I do not know why; it would make a lot more sense (from a safety point of view) to manufacture them in brighter colours.