Let me see.
An alpha particle has a positive charge, the nucleus too. So, the alpha particle has to be distracted like the one which is shown on the picture. So ar, so good. But as the alpha particle has a lower (kinetic) energy, the particle has a lower velocity too. If the alpha particle has a lower velocity, the alpha particle is slower to past the nucleus, that is why the distraction by the nucleus has to be stronger, thus the line which has to be drawn is steeper. I suppose so.