The question "Why do people fall for abusers?" lingers in the background somewhat unanswered.
The answer to exactly why simply isn't black and white. Even if an answer can be found in one situation, circumstances change from one situation to another that may negate the 'general rule'. Often there are additional factors such as mental health issues, childhood experiences or drug use involved that complicate things further, but typically the abuse cycle within any relationship begins completely unnoticed.
It all starts with a charming abuser and an unsuspecting victim. It is important to remember that abusers are generally extremely likable (at first anyway). If they were despicable humans that were aggressive or rude to every person they encounter, they would have a very difficult time creating relationships at all. For an abuser to meet his or her needs, they literally need another person to agree to meet their emotional needs by tolerating ever increasing forms of dominance. Not every person that an abuser meets is going to be the right person to fulfill this unspoken role. A victim tends to be someone that an abuser admires so deeply that the abuser can't help but long to posses the victims every thought.
An abuser is driven by this deep internal desire in a way that causes them to do anything they can to own those admirable qualities. Imagine a toddler who wants a specific toy from an older child who is much bigger. The toddler will whine, cry, negotiate and sometimes even resort to physical measures like pulling or hitting. If these tactics don't work the toddler will continue to escalate to trying other things they've witnessed work in the past. Eventually, they settle on a behavior that usually gets them what they want and that behavior is what the toddler uses anytime they encounter a similar situation.
During the course of their lives, abusers have learned things that work to get their needs met and things that don't work. Similar to a toddler, an abuser is going to resort to using the things they've learned work well to get what they want from a victim. An abuser will lure, manipulate and break down a victim relentlessly over time in order to posses person that they so deeply admire as much as possible.
Most often abuse starts with some form of manipulation tactic that is subtle, this is why it is difficult to say when abuse starts. As each person invests into the connection the relationship naturally deepens. As a result of deeper connection, higher degrees of manipulation are also tolerated. Over time the relationship grows to a point where neither party wants to end the connection or they are ashamed of the things they've tolerated. This is usually the stage when the blatant behavior that abusers are infamous for begins. Obvious abusive behavior doesn't begin until after a victim is willing to tolerate things that normally would be considered unacceptable to both parties.
This subtle manipulation that happens early in the relationship is what most experts are talking about when they suggest victims watch for 'red flags'. The reason experts focus on sharing 'red flag' behavior that seems innocent is to help potential victims can learn to respond to the subtle manipulation differently than they might naturally react. It is much easier to walk away from an abuser before too much is invested into the relationship.
Early in a relationship it is completely normal to believe than even an abusive person is charming or generally likeable. It is important to watch for manipulation, especially if your date has a history of abuse or difficult relationships.