The SAM threat forced the USAF to switch to a low-level penetration tactic rather than high-altitude. They were still using free fall bombs, it just turned out that the XB-70 would barely be any faster or survivable than the B-52 at low altitude while carrying less bomb load at shorter range, so there was no point to the XB-70.

The B-52 was initially saved by the fact that it was "good enough" at low altitude, and was versatile enough to carry many different payloads.

The B-1A was to do the same thing as the B-52, but also have a high-altitude high-speed capability. But discovering that the USSR had developed look-down shoot-down radar in the 70s meant, again, the B-1A wasn't going to be any more effective than the B-52, but would be far more expensive.

It wasn't until the early 80s that long range air-launched cruise missiles (1000 nm+) really became effective and no longer required the bomber to penetrate enemy airspace. Again, the B-52 could carry these and be effective, so it survived.

The B-1B was developed to be a stopgap to the B-2, which focused more heavily on RCS reduction to make it more survivable against AWACS and MiG-31s.

And then of course the B-2 is there for high-altitude penetration missions using either cruise missiles or freefall bombs.

So all that to say, the US maintained the manned bomber leg of the nuclear triad even though ICBMs became more plentiful and accurate. The big problem is, ICBMs (at least in the US implementation) can't move in order to avoid attack. They also can't be recalled once launched, hence the need for the bomber leg. SLBMs overlap bombers in some ways, but subs can be tracked and followed long-term. Bombers are more difficult to track and attack once they're on airborne alert, before they start approaching their targets.