Faded line in spectrogram for a chirp
If you think of the spectrograms as filter banks it might be easier to conceptualize. With a STFT, each bin in the bank has the same time/frequency resolution. With the Wavelet transform, the bins at lower frequencies have higher frequency resolution, which means they must also have lower time resolution.  There is an inherent trade off between time and frequency resolution such that one cannot be increased without decreasing the other.  This can be thought of intuitively as an impulse in the time domain has an infinitely wide frequency response, or infinitely high resolution in the time domain and zero resolution in the frequency domain.