When dh's mother died, he did receive cards from friends and even some childhood friends' parents. It does seem to be a private thing, but I'm surprised if you are good friends with someone that you did not get a card. That could be a personal thing though, like dh and I aren't into cards.
I think it's harder though for people to feel connected to an event in your life if it happens half way around the world. I know my Brit friends and I don't really discuss my family, friends or life in the US, I think because they have no frame of reference for it. I've also found that at least my British friends won't ask questions based on hints. For example, in the US if I had big news I could drop a hint and someone would ask a question to allow me to discuss it. I've notice my British friends don't pick up on these hints and more favour the 'well, if she wants to discuss it, she will' attitude. It's fine, but just meant I needed to adjust. It's along the what's private is private lines. Maybe your friends feel that if you need to talk about it, you will.
I am sorry to hear about your mother. I hope you are coping okay.