the US military (under direct orders of an "oppressive government"), have much more advanced weaponry (including bombs and nuclear weaponry) than what is legally purchasable by civilians.
Nuclear weapons make lousy counter-revolutionary weapons. They are not precise in their targeting and they have secondary effects beyond the blast point.
For example, let's say that there is a militia in upstate New York. The oppressive government nukes that location. For now, we'll assume that the nuclear weapon hits the group's home location and successfully wipes the group out. Now the oppressive government has to handle the fallout from that blast, which will hit Buffalo, Albany, New York City, and even more distant locations like Boston, Philadelphia, and Cleveland. Not to mention places outside the United States, like Quebec and Europe. Sample source for the distance fallout travels.
Worse, you might not hit the target:
- You're using an older ICBM and it just plain misses. There were reasons that the original ICBMs had nuclear warheads. They just weren't that accurate.
- You don't know exactly where the target is and aim at the wrong place.
- The actual target is the people, and people are mobile. They simply move somewhere else.
Even if you hit the target, you might also have collateral damage. Because nuclear blasts are relatively large, they will often hit multiple targets. From Wikipedia:
This is highly dependent on factors such as if one is indoors or out, the size of the explosion, the proximity to the explosion, and to a lesser degree the direction of the wind carrying fallout. Death is highly likely and radiation poisoning is almost certain if one is caught in the open with no terrain or building masking effects within a radius of 0–3 km from a 1 megaton airburst, and the 50% chance of death from the blast extends out to ~8 km from the same 1 megaton atmospheric explosion.
To highlight the variability in the real world, and the effect that being indoors can make, despite the lethal radiation and blast zone extending well past her position at Hiroshima, Akiko Takakura survived the effects of a 16 kt atomic bomb at a distance of 300 meters from the hypocenter, with only minor injuries, due mainly to her position in the lobby of the Bank of Japan, a reinforced concrete building, at the time. In contrast, the unknown person sitting outside, fully exposed, on the steps of the Sumitomo Bank, next door to the Bank of Japan, received lethal third degree burns and was then likely killed by the blast, in that order, within two seconds.
So a person inside a fallout shelter might survive almost a direct hit. Meanwhile, the innocent civilian a kilometer (or eight) away might be killed taking the trash to the curb.
There are reasons why nuclear weapons have not been used more than twice (in the same conflict). They are not terribly effective for military purposes, much less counter-insurgency. Their primary use is damage to civilian targets, which are much larger. Sure, if someone fielded an army, all together, on the surface, a nuclear weapon could devastate the army. But most armies don't concentrate to that level. Certainly an insurgent force wouldn't.
For a modern example, look at Palestine. Israel also has nukes. Yet this completely fails to deter attacks from within Palestinian territory. Because of course Israel never uses the nuclear weapons in response to attacks from Palestine. Palestine shot hundreds of rockets and mortar shells at Israel in 2018. Yet Israeli responses don't prevent future attacks. They barely stop the active attack. And there is some evidence that they encourage future recruitment among civilian survivors of the responses.
Israel/Palestine is an example of two populations that are separate. In most rebellions, the two populations are far less distinct. As a result, some elements of the military may sympathize with the rebels and be reluctant to aggressively attack. Moreover, they might share intelligence with the rebels, allowing rebels to evacuate before attacks. Meanwhile, civilians are stuck in the crossfire.
It is largely irrelevant to say that when there is actual conflict, the insurgents lose. It's certainly true. If things get to the point of actual shooting, the larger group will almost always win. The point though is that the government is generally more reluctant to engage, as they will take casualties on their way to victory.
Insurgencies have been successful in the long term. The US was founded by an insurgency. Vietnam chased out the US. Afghanistan convinced the Soviet Union to leave. Even Northern Ireland has a much more devolved government. But they do this through attrition rather than victories. Casualties and other costs of winning eventually take their toll.