Why is fly by wire better than fly by cable? It would seem like cable would be less likely to fail.

No.

Cable is a mechanical construct, with bell-cranks, pivots, bearings, pulleys, any number of components that can break, stretch, wear out, or fail in other ways.

Fly by wire means you replace all that mechanical paraphernalia with a simple wire. All it has to do is stay in one piece (no reason for it to do otherwise, is there?) and carry the occasional electron or two from one end to the other. Simpler and more reliable.

tanstaafl.