The way I see it is that they are doing this to give the Royal Rumble more significance. How many years in recent memory has the Rumble winner went on to challenge for the WHC and not been the main event? (Obviously last year being an exception)
This time round when the Rumble winner is announced we will know with certainty that he will be challenging for the WWE championship in the WM main event.
This time round when the Rumble winner is announced we will know with certainty that he will be challenging for the WWE championship in the WM main event.