The most likely reason was because there was an obvious correlation between astronomical events and things that happened on earth. The most obvious are seasonal

The most likely reason was because there was an obvious correlation between astronomical events and things that happened on earth. The most obvious are seasonal weather events, which are obviously important for agriculture. Having noticed that these earthly events correlated with certain astronomical events, it was natural to extend this to predicting other events based on astronomical "omens" such as conjunctions, risings, stationary points and so on. It is likely that Babylonian rulers used astrologers when making decisions such as the best time to attack a neighbour.