Why Sensitivity is the smallest signal a receiver can usefully detect, and that depends on the whole signal-to-noise picture, not one part. Front-end gain sets how far the incoming signal is raised above the noise contributed by later stages, the noise figure of that first amplifier sets the noise floor the signal has to beat (noise added early gets amplified along with the signal), and the bandwidth at the demodulator determines how much noise power reaches the detector since noise power is proportional to bandwidth. Tighten the bandwidth, lower the noise figure, or add appropriate low-noise gain up front and the minimum discernible signal improves.
Watch out Noise figure alone is the most tempting single pick because it is the classic sensitivity spec, but gain and detection bandwidth change the measured MDS just as directly.
Sensitivity = signal vs noise: gain lifts the signal, noise figure adds noise, bandwidth lets noise in. All three count.