Wikipedia is somewhat vague on the matter.
What was the tipping point or catalyst for crossbows becoming a prominent weapon in the high middle ages? Can we identify a particular period when crossbowmen (as opposed to standard archers) started to have a noteworthy presence on the battlefield?
This [link] (https://www.reddit.com/r/AskHistorians/comments/1i239w/if_crossbows_had_been_around_since_ancient_greece/) answered by /u/SmilesFTW may be of interest to you, covers nearly the exact same topic.
As a follow up, why didn't the Romans use crossbows? I'd read that the crossbow was a way for settled societies to leverage their wealth into military power and that sounds like exactly something the Romans, especially in Late Antiquity would want.
The excellent old answer by /u/SmilesFTW explained why the Greeks didn't use crossbows much. But few of those explanations seem to apply to the Romans. They were very well versed in siege warfare. They were willing to pay quite a bit to recruit missile troops. Their legionaries were as happy to impale an enemy with a javelin as they were to stab him with a gladius. And they were equipped by the state (I think)
So did the Romans not know how to make crossbows.