Why people don't give up their seat on train?

Once at Week 30 I got on a train and no one gave up their seat then at the next stop when a new seat far from me became vacant a woman who was standing next to the seat blocked the seat and asked me to use that seat. That day I realized that people are different. Some really want to help and some opt to just ignore. I have stood with my belly many times, one time almost 1 hour. Some people just don't see it.