Who said anything about a security breach? Most of the controversy has been about the company influencing elections using data scraped from people (and their friends) unaware of what the data was being used for.
It's often not hard to convince people of something they want to believe.
The fact that people were willing to spend an amount of money that breached electoral law in the UK, and presumably even more in the US, suggests that there was some reason for them to do so. This happened only because experts in this field believed it would influence the outcome of the election.
That's your evidence.