Why does the excitation signal appear, separated, at high quefrencies in the cepstrum?
When using an FFT, an evenly spaced sequence of events in one domain usually produces a strong component in the other domain at a location related to the spacing of the events in the first domain.

A voiced speech signal usually includes a lot of harmonics which are evenly spaced in the frequency domain.  These evenly spaced events in the frequency domain will thus produce a component in the quefrency domain whose location is related to the spacing of the harmonics.  And the spacing of the harmonics in the frequency domain is related to the spacing of the exciter events in the time domain (the period of the fundamental).