Oh sweetheart that's awful, I'm glad to hear you say it's the first and last time 👏🏼👏🏼👏🏼I admire the courage in your convictions. I hope you are safe and I hope you have someone you can talk to about this, maybe get some extra security for home, but right now you just need to let yourself feel how you feel, there some good resources online that can help, including MN.
A lot of the time people don't stop because they think someone else will step in, it's the bystander effect, it's a studied phenomenon, and its shocking and interesting.
I would have interviened, because I have interviened in the past when I've seen or heard someone getting hurt, the last time was on the bus, an elderly lady and small child got on the bus, and ask to sit in the seats reserved for the elderly ect, the chavvy family sat there who'd already been swearing at each other and their kids, just started mocking her accent (she was Indian I think) mocking her, and laughing about it, it was horrible to listen to, I was getting off at the next stop and loudly said to you want my seat, she thanked me and stuff, I apologised to her saying she shouldn't have to go through that, and we had a little chat. My voice is loud and it carries. The mum and dad insulting her had faces like thunder, I just smiled and got off the bus, the way the dad looked at me, if looks could kill, it was the look of pure venom. I just stared back with a little smirk.
Anyway, just make sure you're not physically hurt, as adrenaline can mask it, and just do what you need to do, to calm yourself down. Just remember that this wasn't your fault, but I think you know that already, and please take care 