Wow this is an interesting question indeed. Am wondering what prompted such a question. Let me start by saying, am not a scientist and i do not have the scientific answer for it.
Is there really any logical answer as to why the rain falls slanting as you have put it? Well the one thing i know science has an answer to all questions but there are some things trully that cannot be answered. I know God created the earth the sun and the rain and he knows why the sun sets in the East and rises in the West, it could as well rise in the North and set in South, he knows why the rain falls horizontally and diagnolly right? If i was to give the answer i would probably say its because of the wind, when it blows on the right the rain will fall in that direction and vice versa. I don't think the rain falls without the presence of wind does it? That is something i will have to really look into and watch documentaries on rainfall to find out what truly happens in the earth when it is raining. Rain comes from the clouds in the sky, when it comes down we know it condenses, but when its up there in the middle of the sky and the ground who knows what happens for it to go sideways or cime straight down there is no logical reason am yet to find out.