Why did the historical, observational definition of the AU create challenges for astronomers?

Answer

The Earth's orbit is elliptical, causing inherent distance variability.

Because the Earth's orbit is an ellipse and its distance varies throughout the year, basing a fundamental unit on it introduced inherent variability that complicated high-precision measurements.

Why did the historical, observational definition of the AU create challenges for astronomers?
measurementSpaceastronomysciencesystemunits