The zodiac divides the sun's yearly path into twelve equal parts. The division emerged from Mesopotamian record-keeping, and it postdates the constellations whose names the segments carry.
The constellations came first
Groups of stars along the sun's path were named long before any formal scheme existed, and those groupings are uneven, with some spanning far more sky than others.
Names were drawn from figures that mattered locally, and several are recognisably agricultural or pastoral, which reflects the concerns of the societies that did the naming rather than anything about the stars.
These constellations remain visible and irregular, which is why the modern boundaries astronomers use do not match the equal segments of the astrological zodiac.
Twelve suited the calendar
A lunar month is roughly a twelfth of a solar year, so twelve is the natural number of divisions for a society tracking both the moon and the seasons.
Dividing the sun's path into twelve equal segments produced a system in which a position could be stated numerically and compared with earlier records.
The regularity is the innovation. Uneven constellations cannot support calculation, whereas equal segments can be tabulated, predicted and checked against observation.
Record-keeping drove the standardisation
Long series of observations were maintained over generations, and comparison across such records requires a stable framework that does not depend on individual judgment.
Equal divisions supplied that framework, allowing positions recorded by one observer to be used by another working much later.
The same records underpinned both prediction of celestial events and the interpretive tradition, which developed together rather than as separate enterprises.
The scheme travelled and was adapted
The system moved outward through the ancient world, encountering other traditions of celestial observation and being reworked as it was absorbed.
Names were translated, figures were substituted for local equivalents, and interpretive frameworks were combined with existing ones in each setting.
What remained constant was the arithmetic core, since the equal division is what made the system usable wherever it went.
Signs and constellations drifted apart
The slow shift of the earth's axis moves the seasonal points gradually against the background stars over long periods of time.
Systems that fix the divisions to the seasons and systems that fix them to the stars therefore diverge, and both approaches exist in different traditions today.
The divergence is well known to practitioners in both, and the choice between the two conventions is one of the substantive differences between astrological traditions.