I only read the OP, and I'm going to comment on that one.
When I was a baby, my mom worked, and I was brought to my grandmother, who kept me all day, every day. When my brothers were born, my grandmother would come to our house to watch us, instead of us going to her house (it was easier this way). I think, when needed, my mo will also keep my kids, but...
A couple of years ago, my mom got cancer, and so she is now limited in the things she does + gets tired very often => maybe/ probably this will limit her in keeping my kids.
In case of my mom, she lived close to her parents, as opposed to her sibblings. So, there weren't requests from my aunts/ uncles to my grandmother to keep their kids too. If I live close to my parents and my brothers live close to them too, there is a risk my mom finds herself with a whole bunch of kids, which she might not be able to handle all at once. It can be she'll say she won't have them at all, to not favor one above the other.
Also, in my grandmother's "age", it was common for a woman to stay at home, and raise her kids/ grandkids etc. So, for my grandmother, this was the "normal" thing to do. Times have changed in the meanwhile, and many women I know that are a "new" grandmother now, say: have raised my kids, I have done my share, now it's time to enjoy life. And I can only agree.
Do I still hope my mom will keep my kids if I ever have them? YES! Will I be mad at her if she can't? No. That's why they invented babysitters.
Also, maybe your mother doesn't approve/ agree with the way you educate your kids and doesn't feel comfortable in "taking over the educating" while your children will be with them? I have felt like that as a babysitter, and because of that reason, I have stopped a regular babysitting job. I couldn't handle the way the parents raised their kids, I couldn't "manage" those kids that way, so i'd rather not keep them. It's just an idea, so no flames, no nothing

Maybe she is scared she will do something wrong?
I don't know how old she is, and how old your kids are, but maybe she forgets things once in a while, and is afraid to forget things while being with your children.
I wouldn't be mad at her, that doesn't help. Try to find a solution, like a babysitter.!!
ETA: about wanting grandparents to get close to their grandchildren: it's great if it works. My brothers and I have a great relationship with my grandmother from mother's side. Hey, if she didn't come with us on vacation, we didn't want to go either!

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But we don't have a realationship with the other set of grandparents. Never even saw tehm, until recently (long story, will not tell it now, but I saw the other grandmother twice last year, don't feel the ened to see her again)
I don't feel like I really missed out on much. yes, one part of the family isn't involved in my life, but the other part made up for it big time!!!