Anticipation

I remember first hearing the buzz about an alien prequel being touted around with Scott's daughters boyfriend lined up to direct? But Fox wanted Scott to direct it instead. My question is this: Why didn't Scott want to direct it in the first place? He goes on and on about how glad he is to be back in to making a sci fi film...so why be dictated by Fox? I may have missed some things on here so bear with me. The running time is still two hours and the rating is R?