The 12-tone scale allows for more pleasant sounding harmonics than any other equal-tempered scale consisting of a small (manageable) number of notes per octave. Even those harmonic combinations in the 12-tone scale are just close approximations. This is a matter of the mathematical ratios of frequencies closely approximating simple fractions such as 1:1.5 and 1:1.333.

So it was fairly clear from the start that everyone needed to adopt a 12-tone scale.

How the exact pitches were settled upon, I have no idea except that eventual standardization would be critical to allowing musicians to play together, and was therefore not really optional. They had to standardize.