For most of recorded history, observing the sky and interpreting it were a single occupation practised by the same people. The separation is comparatively recent and had specific causes.
The same skills served both purposes
Predicting where a planet would be required careful observation, sustained records and considerable mathematics, and interpretation required exactly the same outputs.
An interpretive practitioner needed accurate positions for a given moment, so improving the tables served both activities at once and there was no conflict of purpose between them.
Much early mathematical astronomy was developed and funded because of this demand, which is a matter of historical record rather than of interpretation.
Patronage supported the combined practice
Rulers employed practitioners of the sky, and the interpretive service was frequently the reason for the appointment and the source of the funding.
Institutional support of that kind maintained observatories, instruments and continuous record-keeping over periods far longer than any individual career.
The observational archives built under such arrangements were later essential to work that had nothing to do with interpretation.
The distinction we draw did not exist
Practitioners did not experience themselves as combining two disciplines, since the idea that celestial positions were meaningful was a shared background assumption.
Terminology reflects this, since the words for the two activities were used interchangeably in several languages for a long period before the modern distinction was settled.
Projecting the modern division backward misdescribes what these people thought they were doing and why their work was organised as it was.
The separation had several causes
Changes in physical explanation, in institutional structures and in what counted as evidence all contributed, and the process was gradual and uneven across regions.
Some practitioners continued both activities well after the split had begun elsewhere, and the boundary was drawn differently in different places.
Describing this accurately means avoiding a single decisive moment, since the change was a shift in what questions institutions were prepared to fund and publish.
What each retained
Positional astronomy kept the observational archives, the mathematics and the instruments, and continued to develop all three for purposes that no longer had any interpretive component.
Astrological traditions kept the interpretive frameworks, the technical vocabulary and the calculation methods, and continue to use position data produced by astronomers.
The two remain connected in this practical way even though they now answer to entirely different standards and communities.