I have no neices or nephews. My daughter is the only child in this generation. My brothers saw her at family dinners, usually held during holidays, but were never involved in her life otherwise.
My parents had neices and nephews. Our families were close, mostly living close to each other. I am still close to a couple of cousins. Those cousin's were/are not close to my daughter.
Our family is not as close now that our parents' generation is gone.
My brothers, 3 of them, have no children, don't know how to relate to children. I never thought about this as a problem. People are who they are. I don't expect them to change their personalities or interests for my daughter.
My brothers aren't married. My older cousin's are married. It's been their wives who have kept us close as adults. Wives have asked about my daughter. They have siblings with neice and nephews. They are naturally more involved with them.
Sounds like you want your brother-in-law involved in your children's lives. Why? Why would you even question why he's not? He is who he is. That's why. Love him. Accept that and move on. Expose your children to other adults who want to be involved with them. I do not think my daughter has missed out on anything by having very little time with her uncles.
Another thought. Sounds like you and your husband are not close to his brother. Why would you expect him to spend time with your children? Why is it important for your children to be interested in your children?