Some of it is because they weren't exposed to "tipping" for large parts of their life.
I know several from work that had never been in a restaurant until they were adults.
If you aren't around it then you aren't as aware of it, the importance.
The best tippers come from people that have relied on tips in the past as part of their income.
Bad/good tippers are often associated with familiarity, upbringing, income levels and so on, it's not just about skin color.......
For example Canadians don't tip well either.