That is only one condition among the conditions that had to be satisfied by the sampling rate, and there are an infinity of multiples which satisfy this condition, so this condition is insufficient to determine the choice of the sampling frequency.

Another condition that had to be satisfied by the sampling frequency was to be high enough in comparison with the maximum bandwidth of the video signal, but not much higher than necessary.

Among the common multiples of the line frequencies, 13.5 MHz was chosen because it also satisfied the second condition, which is the condition that I have discussed, i.e. that it was possible to choose 13.5 MHz only because the analog video bandwidth had been standardized to values smaller than needed for square pixels, otherwise for the sampling frequency a common multiple of the line frequencies that is greater than 15 MHz would have been required (which is 20.25 MHz).