Why no children?

There was never a reason given, and Betsy Brandt can only speculate herself:

I think Hank and Marie would love to have those kids. I always felt like there was this undercurrent of sadness when they have Skyler and Walt's kids in their house, because I think they probably wanted kids and couldn't have them. I don't know if they talked about it or not, if it even got that far, or it just didn't happen and that was it and they just left it. It didn't work out that way.

That's her answer to an interesting question by Sean T. Collins that might hint at a possible motive the writers had for this situation:

Hank and Marie have no kids of their own, and no family to speak of other than Skyler, Walt and the kids. The spark for Marie's attempt to take Holly was finding out about Walt and Skyler, of course, but it looked like she and Hank were dealing with a lot of unspoken pain during that scene, too.

Dean Norris also touches on that theme of Hank and Marie being alone and having no family in this interview:

This season, when Walt and Hank square off in the garage, the look you give him is chilling. What were you thinking about to summon that expression?

We went through a little rehearsal on that scene. At first, it was much more violent. We realized that rage was just a part of it. He also feels hurt and fear. He feels betrayed by his own brother. Marie and Hank have no kids, you never see me talk about mothers or fathers or grandparents. Walt's his brother. And obviously, Hank feels like he's the tougher, older brother.

In another interview he says:

Betsy Brandt and I have talked about that, too. We’ve always wondered. I don’t know why. I always thought it wasn’t that that they didn’t want kids, it was that for some reason or another they couldn’t have kids, because yeah, it does seem likely that they would. Maybe you can see that longing to have had a child when we have baby Holly in our house. We don’t linger on it, but just a tad of “It would have been nice to have our own.”

For me, that brings to it that “She’s all I’ve got, and I’m all she’s got” dynamic. That’s something that we can work with and use. I think that’s part of the reason each of us puts up with the other’s bullshit. [Laughs] I never understood the Walt/Skyler attraction myself, but I really understand that Hank and Marie would go into battle for each other.

However, Betsy Brandt seemed too think at one time that there were plans to elaborate on this subject:

Why don’t Hank and Marie have kids?

I want to know too! I’m fine with them not. But this is one of the things I love about Breaking Bad: The things we don’t say, we deal with them without straight-on dealing with them. It will be addressed, but not directly addressed, in an upcoming episode.

But nothing came of that, AFAIK.

Reviewers also speculate about this subject:

It’s always been interesting to me that Marie and Hank don’t have any kids, since they clearly like their niece and nephew so much. When Marie would babysit Holly, she’d joke that she was going to keep her forever. This is total conjecture but I think that they’re childless not by choice but because they couldn’t have any. Whether it’s because of Hank or Marie, it explains a lot of the relationship they have with Walt and Skyler. Hank would see his being unable to have a kid as a symbol of not being a real man. Hence the bravado. If it’s Marie who couldn’t have them, that inability seems to have manifested into a case of O.C.D. and steady need to diminish her sister. I’m sure she and Hank were trying to help when they took Walt Jr. and Holly in, but that doesn’t mean they didn’t feel a jolt of victory about being the ones whose marriage was healthy and intact while Skyler and Walt’s was collapsing. Walt’s not the only character who sees things in terms of winning and losing.

Another reviewer has a different view:

The dynamic between Skyler, Marie and her baby is by far one of the most peculiar of the episode—Marie, without children of her own, has always taken on a maternal role with her sister’s children, whether Skyler warrants it or not. Hank and Marie’s childless marriage is a plotline that isn’t at the forefront of the show, but has always lingered in the background.

“You are not leaving this house with my daughter. Give her back to me!”

“I’m trying to protect her.”

“Protect her? How dare you, I am her mother!”

Marie fiercely believes taking Holly away from dangerous and unfit parents is what’s best for her. It’s not out of spite or her incessant need to one-up Skyler, it’s a sincere concern for an endangered child. But is her attempted kidnapping merely a result of genuine concern, or is there a tinge of jealousy in Marie? Why is it fair that Skyler gets to be a mother when she’s not even fit to parent? Marie may feel that she deserves to take care of Holly, not her suicidal, hysterical sister.