Why use one larger power cable instead two parallel cables for motor?

As far as running two cables for reliability, the problem is that the failure of one parallel set of conductors reduces the available current to the motor, preventing it from starting. If you are seeking parallel redundancy, each triplex or set of three phases of conductors needs to be sized to carry the full load.

Also, in most countries that follow the guidelines of the NEC, CSC or IEC, the largest standard conduit is 4 inch. 500MCM (kcmil) is the largest size of triplex that can fit into a 4 inch conduit. 750 MCM is not produced in large amounts and costs more per unit length in material and labor. The bending radius of larger conductors is greater and more difficult to pull through raceways. The terminations are also difficult in a tight enclosure space.

When parallel sets of current-carrying conductors are used, the NEC requires that the neutral conductor in each parallel conduit be sized for 100% of the circuit amps.

The availability of common conductors up to 500 MCM (kcmils) is much greater than a non-standard conductor or cable. Most installations use conduit or other raceway to route the conductors or cables from the power source to the load. Larger conductors are more difficult to handle or pull through a conduit and they require a greater bending radius.

The common conductor sizes are in greater supply and usually kept in stock by distributors. This results in a lower unit cost and a greater availability versus a custom size that requires a longer lead time to manufacture and ship to the customer location.