The agreement was apparently for Cameron to write something up and Scott to do the producing side of it. We know from 'Prometheus' that he liked to have an on-going oar in the water, in so far as wanting to bat ideas to and fro, which didn't happen here. But besides all of that, as a prospective producer, he should have been consulting about stuff like budget requirements, prospective filming locations and other things for the pre-production phase.
I simply think he either just wasn't all that invested in it or is easily distracted.