Why Decimation throws away samples to lower the sample rate, which lowers the Nyquist limit to half the new rate. Any signal content above that new limit folds back down into the passband as false lower-frequency signals (aliases), and once folded it cannot be separated from real signals. So a low-pass (anti-aliasing) filter must run before the samples are discarded, removing everything above half the new sample rate.
Watch out The idea of notching out the sampling frequency misses the point: aliasing comes from content above half the sample rate folding down, not from the sampling clock itself appearing in the data, and a single notch would not stop it.
Decimate = lower Nyquist. Filter first, then throw samples away, or highs fold down as fakes.