The ASCO Annual Meeting is the real deal. In my opinion, it is where the truly important and clinically relevant oncologic research of the year is presented. You'll note that many major RTOG trials have their results presented at ASCO first.
So, long story short, they have a higher standard and getting an abstract accepted is tougher. Also, since they technically have a higher focus on Med Onc, a Rad Onc abstract probably has to have a pretty steep impact factor to be seriously considered.