A few thoughts that I had - not sure if any apply, but maybe worth considering:
1) As an introvert, you are more likely to wait for people to approach you first. Manipulative people are unlikely to choose someone that they feel will call them on their stuff or see through them immediately. As a P, you are more likely to be open to seeing what someone is all about rather than going in with preconceived "shoulds" beforehand.
2) If you have generally not had to deal with manipulative people in your formative years, you may measure people's behaviour by what you would do in that situation and therefore be surprised when they don't act that way.
3) If you have had to deal with manipulative people, this pattern may feel safe and familiar even though it is terribly drama inducing.
4) When they are young adults, most people tend to be less experienced with a wide variety of people (no base to measure new behaviours against and determine what they mean) as well as being less confident asserting oneself early on before patterns become well established.
5) When you see people treating others poorly, it is only a matter of time before you yourself will be treated poorly by them. Their treatment usually is borne out of their own insecurities, lack of trust, and need to look out for their own interests, but it is terribly destructive. It takes time to learn this.
6) You may be drawn to trying to figure them out, especially if they have attractive qualities, act inconsistently, seem mysterious, do the push-pull etc.
7) They may make you feel significant, attractive, wanted or welcomed. This is compelling to most people. It takes time, personal experience and seeing the person in a variety of contexts to determine what kind of character and past behavioural patterns they have. If you are too open initially, you can be pretty entangled by that point.
8) You may have an insufficient amount of support from other people in your life. That makes you a beggar instead of a chooser for what kind of treatment you accept or what people you allow into your life. Somebody seems better than nobody, even if it is less than ideal.
9) They have attractive other qualities which make it hard for you to believe that they are truly a bad person or that they would want to harm you.
10) They make you feel as if you can help them and may make you feel needed, guilty or obligated.
11) You feel selfish or rude or unsure establishing firm boundaries. (I've found it useful to have some kind of a measuring stick for behaviour that I will/won't accept and then script myself ahead of time for situations I'm likely to encounter so that I'm not put on the spot or agree to more than I'm comfortable with or is in my best interests.)
Certainly in my own life, several of these have been very true. It's taken time to learn how to effectively draw firm boudaries. Over time I've seen that by establishing those boundaries, it provides security for the other person, protects me, and allows me to treat them more kindly over the long term because they have not been able to do any actual harm to me. You are making your verbal and non-verbal communication match each other and in doing so, are sending out clearer messages about what you will or won't accept. I've also found that by recognizing patterns earlier on, it is easier to avoid becoming deeply entrenched in unhealthy relationships or friendships.