It's not that yellows > legendaries, it's that legendaries and yellows follow the exact same formula, but you get hundreds/thousands more yellows than you do legendaries and so it's much more likely that you will find an item with both the right stats and a high roll for those stats, legendaries though will roll more of that same stat. For example the max primary stat roll on a yellow pair of bracers is 415 while legendary can go over 500. In addition to that you obviously have the unique stats and bonuses that some legendaries give.
Legendaries are definitely considerably better than yellows, if you get the right stats and they roll a good ammount. I'm stuck using yellows for most of my slots due to that.