1. Which is why they drive Fords in Ohio.
2. That is why they drive Fords in Ohio.
3. Which is because I got up late.
4. That is because I got up late.
I came across the 1st sentence today, which I hadn't seen before, and wonder 1 and 3 are same as 2 and 4 in meaning.
The first is not a sentence but is a dependent clause looking for some words to hang with so it can become part of a sentence. Number three is similar. Numbers two and four are fine as sentences, but we are missing some critical information. (Why do they drive Fords in Ohio?) I can imagine number four being something like:Did I answer your question?
I was late for work today. That is because I got up late.