CUBIC (red) uses packet loss as the congestion signal — additive increase until loss, then multiplicative decrease. On a slightly lossy link it spends a lot of time recovering, losing throughput.
BBR (green) estimates the bottleneck bandwidth and round-trip time, paces at that rate. Doesn't mis-attribute random loss to congestion. Throughput stays much higher on lossy networks.
Slide the loss rate to see how each algorithm reacts.