Why is a good question because it doesn't make sense. So why? I found my way here years ago, and worked through some issues. I am glad you are here and starting to let the pain out. So many of us will understand how you feel, even though the details of your situation might not match up with our own.
I am glad you found love again. Your previous marriage issues are not your sons' business. I wouldn't make what they think about your business, your business either. It is their problem how they hang on to the past and make themselves miserable. Sounds like you decided not to be miserable, but would like to have their involvement for the grandchildren's sake, is that right? What would it be like to clear the air with your ex, and find out what he knows about your sons' feelings? That would give you insight at least. I would also wager your ex could put in a good word for you to straighten things out with your sons.
I would not let your sons' mess with your current relationship or happiness. They are loved, but you have a choice whether to let them mess up your life or not. I would choose to let them be, and do what grandmothers do. Write postcards, call on birthdays and holidays. Ask to speak and see your grandchildren. In other words, keep trying, keep loving, and let them stew in their own juices!
I am not the wisest one here, so if this all sounds way off, just take the hugs that I am sending you as I mean well.