Selective Attention.
A bottleneck restricts the rate of flow, as, say, in the narrow neck of a milk bottle. The narrower the bottleneck, the lower the rate of flow.
Broadbent's, Treisman's, and Deutsch and Deutsch Models of Attention are all bottleneck models because they predict we cannot consciously attend to all of our sensory input at the same time.
This limited capacity for paying attention is therefore a bottleneck and the models each try to explain how the material that passes through the bottleneck is selected.