First of all, "unbreakable" steel cables break if overloaded, they also rust and develop fangs that wound terribly. Their strength declines as individual wires rust and break.
Straps also break if sufficiently abused but they are much safer.
I would imagine it's a combination of product price, ease of handling (No jags ripping your fingers), ease of use (cable doesn't always roll up on the drum evenly), and aesthetics.
Most are happy with the straps. Some die hards want their cables. Choose what you want and "roll" with it!