The arc ping info alone divides the probability equally to 50% on either end of the arc. The reason given for searching the southern route into the ocean is based on the two images of something floating out there. Malaysia doesn’t want to deal with pursuing the religious and political overtones of the northern route. The southern route is an acceptable way for them to just let the missing plane fade into a distant memory.
The first reason for looking south is because India and China and others would have seen that plane on radar once it headed over land and all of them say that it did not show up on radar.
The second reason is that ACARS would ping if the landing gear went down and there was no ACARS ping for that. So the plane either crashed in Kazakistan or took the southern arc and ditched in the waters.
The third reason is that the last engine ping at 8:11 puts it within the perview of US radar stations in Afghanistan and those stations saw nothing.
Thus by process of elimination they are looking south.
The key moments in the flight would be between 2:11 and 4:11.
Note that from 3:11 to 4:11 the plane flew along the same arc.