A lot of the devil here is in the details. You need long term life support that Dragon hasn't proven yet. Deep space communications on the transmit and receive side. Abort scenarios for most or all phases of flight. All of this is untested and requires a huge amount of engineering and manufacturing work. Since this will likely share the same design as the NASA-contracted capsules, these systems will need to pass muster with them. And they don't cut corners.
Minor quibble: you don't need deep space communications. The Moon is laughably close. I am quite confident that we can easily make contact with the existing unmodified Dragon capsules by using something like one of these bad boys: https://www.nasa.gov/sites/default/files/goldstone.jpg
Of course it's laughably overkill for the purpose -- amateur radio operators using much smaller equipment can already send and receive signals to each other by bouncing them off the Moon, which has four times the signal attenuation owing to the doubled distance plus a good deal more in losses from the poor reflectivity of the Moon to radio waves.
