Maybe they don’t realize how close you were to your grandparent?
Maybe they had in mind to ask you about it but it slipped their minds because they were busy and would feel weird asking now, so far after the fact and having waited so long?
Maybe they feel that what they did really was enough, and have no idea that they left you feeling like you do?
I’m sure there are many different possible reasons, but that still doesn’t really help resolve it.
I actually also find myself right now feeling upset at someone for something they didn’t do, and wondering if I’m being too sensitive. I’m still trying to figure it out, but as hard as it is to accept, I’m starting to think that maybe I am. Although I am pretty certain of the other person’s reasons (forgetting – which can be one of the most hurtful reasons), I do have the obligation to judge others favorably, and who am I not to fulfill this Torah obligation? It’s really hard and I’m still trying to figure it out, but as of now I think that whatever the other person’s reasons, this is not one sided and I have my own responsibilities to pay attention to here. Maybe Hashem made them forget on purpose so that I can be tested in how good I am at really fully and truly judging favorably, forgiving, and forgetting? You never know.