The reason is there could be a "freeze" crack on that side of the block behind the intake and exhaust manifold that is difficult to see because of them. This is a large area which is prone to this if there was not anti-freeze in it. I believe it was mentioned previously that it had a freeze plug pushed out. Freeze plugs are actually casting plugs, and if you get lucky they will come out without other damage. There is no such possibility for relief on the left side of the block.
