This is probably to be explained from the extreme rapidity of the Mesopotamian rivers, on which sailing boats are still uncommon.

The unfailing strength of rowers was needed in order to meet and stem the force of the currents; and this strength being provided in abundance, it was not thought necessary to husband it or eke it out by the addition of a second motive power.
Again, the boats, being intended only for peaceful purposes, were unprovided with beaks, another invention well known to the Assyrians, and frequently introduced into their sculptures in the representations of Phoenician vessels.

[PLATE CXXXIII., Fig.

5.] In the Assyrian biremes the oars of the lower tier were worked through holes in the vessel's sides.

This arrangement would of course at once supply a fulcrum and keep the oars in their places.