Where I grew up it was the wife's job to remember family birthdays, send Christmas cards, write letters to family and friends at a distance, so I naturally carried on doing so.
Perhaps you did the same? If so, either your DILs were brought up differently or are of the "Their your family, I write to mine" school of thought that explains why your DSs don't write.
Some people too assume, if you are not at home on your birthday that's because you don't want to celebrate it.
If you can let it slide, do so. If not tell your son that you were hurt by him ignoring your birthday.