Actually in this case there is no reason for Ronaldo to leave especially for Roma.
Even though it's the worst possibility now that Ronaldo has moved from MU, it's clear that his choice is not Roma.
The reason is quite clear, he wants to gain experience and doesn't want to be separated from the Champions League and Roma certainly can't do that so why would he leave if he still can't be in the Champions League.
If Ronaldo leaves for Roma (although chances are slim ) he might not be too worried about playing in the champions league. He might be yarning for a different adventure in the Seria A. So far Ronaldo has achieved a lot that one more season of not lifting the CL trophy might not mean much to him. He could have something bigger he is hoping to achieve with José Mourinho. Perhaps if Ronaldo remains in united he won't still play in the champions league
Instead he left because he might be looking for a newer experience and obviously the Champions League is still a target for Ronaldo.
Now linked with Ronaldo there are also some strong teams like PSG and Chelsea. Unless Ronaldo wants to be at Roma like Lewandowski wants to be at Barcelona it's likely to happen, but if not then there's no reason for him to be at Roma.