You fail to grasp just how mindboogling fast space is. A shrapnel bomb launched in the opposite orbital direction would hit that target at about twice it's orbital speed. For LEO that means impacts of around 15 km per second).
And considering kinetic energy is m*v2/2,..., that matters a lot.
A 10 gram object at that speed has the same impact as a modern tank gun firing a 2.2 kg kinetic energy penetrator.
You fail to grasp that at this point people are doing navigation and aiming by eye, pencil, and notepad. Speeds are low, ships are designed with the expectation of damage, and the bombs would be being launched from spacecraft maneuvering so that they can actually hit the target, which means trying to match it in speed, or at the very least not going full speed in the other direction that its going. It is very hard to do space fighting when you are going so fast you only see the other ship for a few seconds, you know?
A cluster bomb would be lucky to actually hit anything with a large fragment, much less come down on someone from dead in front of them coming full speed in the other direction.
1) Speeds can't be low. If speeds are low, you're not in space.
2) You have a vehicle undergoing re-entry at high speed; with several vulnerabilities (bomb bay). A single hit will be sufficient. As said before, each gram of bomb material has a massive energy. As much as 25 grammes of TNT per gram of material. It'll cut straight through any armor, or at the very least damage it sufficiently.
3) The bomb would be launched using it own rocket, no humans allowed. It's the functional equivalent of an anti-aircraft flak shell, no advanced systems required. This allows a greatly increased payload. The single use Mercury rocket weighted 1.4 ton, and had no real orbital
4) You can't match speed with any target intending to bomb the continental United States. See; the target launches from Germany intending to bomb the United states. You can try to intercept it over the Atlantic, but then you'll be going the wrong way. Reversing course mid-flight is physically impossible so that doesn't work. So the only thing you can do is launch after it flies over (so after the bomb is dropped), when the target has either already re-entered to bomb or is just re-entering to land in Germany. In both cases, getting enough speed will be hard, and the encounter will be in atmosphere.
Of course, we can station forces in space to intercept any upcoming craft, but then we would need to have craft over the entire orbit, in range to pounce on an enemy within half an orbit. At that point, the range of a fighter is smaller than the range of our shrapnel cloud.
And when the enemy launches a space station, it's trajectory is predictable, so a shrapnel bomb can take it out. And thanks to the way tech here works, the shrapnel bomb will hit them before they find out they need to evade.
You simply can't beat a kinetic kill vehicle in space.
And also, despite the vastness of space there's a very limited re-entry window that can be used if you want to attack the continental United states. So, calculations are no problem. We know where they're going to be, and they have no option but to pass through that space.
However the current bomber is still sub-orbital and doesn't really reach space, hence, artillery rockets on a B-29.