Why does cable length matter so much when source is high impedance?
The problem is the cable's capacitance, which is proportional to its length. This capacitance works in conjunction with the high source impedance of the microphone to create a low-pass filter that can severely attenuate the higher frequencies that you're interested in.

For example, if your microphone has a 100K output impedance, and your cable has 1000 pF of capacitance, together they create an R-C time constant of 100 &micro;s, which corresponds to a cutoff frequency of about 15 kHz. Anything above this frequency will be attenuated at a rate of 6 dB/octave. Longer cables mean more capacitance, reducing the cutoff frequency proportionally.