[Right_Wing_Madman]

Vespasian feared the machine would exacerbate unemployment in a society already overrun by idlers and slaves.

Also, Professor Peter Green points out in Hellenistic History and Culture:

It's not so much that slaves were available, which indeed they were. No, the ruling classes were scared, as the Puritans said, of Satan finding work for idle hands to do. One of the great things about not developing a source of energy that did not depend on muscle power was the fear of what the muscles might get up to if they weren't kept fully employed.

[SunkenCiv]

As much as 40% of the city were slaves, so, good idea to keep them busy.