The phrase "main impetus" refers to the primary force or driving factor behind something. It represents the main reason or motivation that pushes or inspires an action or decision. Full definition
High's main impetus for signing onto the program. (npr.org)
The only detail that is clear is that they're being chased, and is the player's main impetus for progress. (bitcultures.com)
If Home Depot were to make an offer, one main impetus would be to keep XPO out of the hands of Amazon — which the home improvement retailer believes has also considered buying the logistics company, the source said. (cnbc.com)