Why a high number of harmonics means a lower bitrate?
It is unclear what you mean by "implying" a particular bit rate, and what exactly you envision bit rate meaning.

If you wish to accurately measure a signal by sampling it at regular points in time, then the minimum necessary sample rate (not bit rate) is dictated by the frequency content of that signal.  In fact, the absolute minimum sample rate is twice per cycle of the highest frequency of interest.  This is often referred to as the Nyquist rate.

Any arbitrary repetitive signal can be decomposed into a sum of sines with specific amplitudes and phase shifts.  The period of these sines is the basic repetition period of the composite signal divided by all positive integers.  These are called the "harmonics" of the signal, with the first harmonic having the base period/1 sometimes called the "fundamental".

Therefore, sampling a repetitive signal is the same as sampling all the sines that are added up to make that signal.  The Nyquist rate applies to each of those.  Therefore, more harmonics, which means the sine components will to go to higher frequency, requires a faster sampling rate.  It is the highest frquency of interest that dictates the minimum sampling rate.  More harmonics means a higher frequency of interest.