it's because whereas the repulsion between protons acts over a comparitively large disance, the force keeping the nuclei/neutrons together is the strong nuclear force-this acts at very short distances only, this is why, as nuclei become very large, the repulsive forces between protons acts on every proton in the nucleus, whereas the strong nuclear force only affects its nearest neighbours, hence why as atoms become larger, there is an increasing amount of neutrons needed.
I haven't revised tyhis yet so most of it is probably wrong haha, but that's more or less it. if you google it you should find out more!