One factor which should engage our enthusiasm for the moon's resources more highly than for those of Mars is that lunar resources are sufficiently nearby that their utilization could have returns to the terrestrial economy. Surely this is vital for getting the space enterprise off the ground, given that every investor currently in existence lives on the planet Earth. The usefulness of Martian resources for the purpose of constructing habitats for living on Mars is indisputable. These are all products marketable to Earth, as opposed to hypothetical future Martian investors. Mars will never be in as good a position for export as the moon due to the combination of its greater distance and steeper gravity well.
Export is vital for balance of trade, and hence economic viability.
When Prospector detected water ice at the lunar poles, the usefulness of this water for lunar surface operations was obvious to everyone. The distance involved is much greater, but in terms of delta-V, the moon is a better source. In addition to having a shallow gravity well, the moon is also airless. This opens the possibility of electromagnetically launching lunar material horizontally off the surface and into space. O'Neill began building models of a device he named the mass driver.