The reason is that for a Loxleo device, the oxygen is already in space. This allows a tug with high ISP ion engine to deliver it to the Lagrange point for a reasonable cost in propellant. You could launch from the moon's surface, but I suspect that the construction job would be more difficult and expensive than a construction site in LEO.
With a Loxleo device, you could do away with 80% of the mass that has to be lifted from the Earth's surface. The remaining 20% would be methane. Both will travel together on the tug to EML-2 gateway for the purpose of providing reaction mass for eventual trip to the Mars system. For every 100k lbs of fuel located there at the gateway, and used from the gateway, a little more than half that in mass can be powered to the Mars system. Therefore, to deliver 50k to Mars, you'd only need to launch 20k in mass, plus the mass the tug used in getting the fuel there. The tug shouldn't use much since it is a high ISP engine. ( about 5% of the mass)
If you save the mass launched from the ground, this means fewer launches in total.
Now, if you tried locating it on the Moon's surface, you'd have to build a device that can get the oxygen to the Lagrange point from the surface. To land there using a spacecraft will defeat the entire purpose. You use up as much fuel to get to the surface as you could bring back from the surface. Therefore, you must build something like a gas gun or a mass driver on the lunar surface. This may prove to be difficult and expensive. Easier to just get the gas from the upper atmosphere of the Earth and transporting it there with a high efficiency ion engine.
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