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

it doesn't really matter for me. An elderly gave up her seat to me in a crowded bus while I'm 4mo preg with my 3.5yo. I insisted that she sit while I get my girl to sit beside her (empty seat) while I stand and hold on to the handrail. If I'm really unwell I will tap on the shoulders one by one to ask if they are okay to give up their seat because I'm really unwell. not everyone is considerate, but I believe there will be kind soul amidst the black sheep somehow..