We only sample the signal for a certain time.
We cannot see slow changes in the signal if we don't wait long enough.
In fact we must sample for long enough to detect not only low frequencies in the signal, but also small differences between frequencies. The length of time for which we are prepared to sample the signal determines our ability to resolve adjacent frequencies - the frequency resolution.
We must sample for at least one complete cycle of the lowest frequency we want to resolve.
We can see that we face a forced compromise. We must sample fast to avoid and for a long time to achieve a good frequency resolution. But sampling fast for a long time means we will have a lot of samples - and lots of samples means lots of computation, for which we generally don't have time. So we will have to compromise between resolving frequency components of the signal, and being able to see high frequencies.
