The practice of taking slaves when sacking a city was almost like an unwritten rule.
That did not happen when Alexander took Adhan or Zanzan because those cities had their own unique circumstances.
One was the capital and given Amenheraft was about to attack, turning the citizens into slaves was a moot point.
As for the other, well that was Alexander's capital, and turning the citizens into slaves was also a moot point.
But Thesalie was different and upon taking it, many citizens were captured to be slaves by the slaves.
In fact Alexander was sure that if he were to return to his camp right now, he would not only find every tent overflowing with gold, jewelry, fabric, etc. but also many beautiful girls and even boys.
The soldiers would enjoy them for a few days and then sell them off to slave traders for a profit.