It's not something we like to talk about, or even admit. If only a small portion of stepchildren feel close to their stepmother (20%), how many feel the other extreme of hating their step parents? I believe it is a small percentage. Yet, it's worthy of discussing. There are 2 main reasons I have come across for stepchildren hating their step parents.
1) You're perceived as the "other woman" or the "man who came between my parents". This may be obvious if the kids know their dad is cheating on mom (or mom is cheating on dad). Children want their parents to stay together. If somebody else enters the picture, and then suddenly (seemingly to the children) their parents get a divorce, you become the scapegoat for all the discontents and adjustments they will face in the coming years. And there are many. Even under the best of circumstances, it's not easy for children to understand why their parents split up. It's a difficult adjustment regardless. The cheater just made it easy for them to name a reason, and they WILL take it.
I learned this first hand because this is the primary reason I hated my dad's girlfriend. There was no doubt in my mind that she was to blame for everything I didn't like about my life after my parents' divorce. My mom shared the same feelings for her and wasn't shy about vocalizing it, which validated and reinforced my feelings. Indeed, if she hadn't come along, they would have lasted longer. Cheating is a symptom of marital problems, not the cause. They likely would have divorced anyway, but nobody explained that to me as a child.
Cheating is not the only way to be labeled as an interloper. Surprise! Children often hold onto the illusion their parents will reunite.If one of the parents has not gained closure (i.e. moved on emotionally) and holds regrets or resentment, she won't be ready to see her ex with a new woman. Hence, she may subtly (or not) pass those same feelings to her children.
If the break-up seems amicable by outside appearances, the children are more likely to hold onto the illusion of a reunion. This could easily be amplified if both parents stay equally involved with the kids. If this pattern changes when a new girlfriend comes on the scene, (i.e. dad is suddenly spending less time with them because he's spending time with his girlfriend), they may not be too open or accepting of you, the stepmom.
2) You've been criminalized by one or both of their natural parents, either intentionally or unintentionally. Criminalized may seem like a harsh word, but that's exactly what it feels like if you are a step parent who already has odds stacked against you.You're trying your best to fit in and be the best parent you can be while not stepping on any toes. You want nothing more than the best for the kids and proceed accordingly. Wait! Dad disagrees - out loud, no less. He then reverses the punishment you just gave in favor of what his son wanted. You now look like the bad guy. Whether the arguing is out loud or private, if it's obvious to the kids, you become the reason dad is unhappy. The more dissension between you and your partner, the more likely the children won't like you. If dad doesn't understand you, and doesn't look favorably on your parenting, he can't defend you to his kids, yet alone express happiness or gratitude for your contributions.
I mentioned earlier that a parent who hasn't gained closure from the divorce may hold regret or resentment. I've seen this more with women than men and I'm not sure why. She may even want to get back together. In this case, she won't be accepting of her ex's new girlfriend, regardless of how long they've been separated or divorced. If the ex-wife criminalizes you as a stepmom, your step kids may feel disloyal to mom by liking you. Dad has to address this directly with the kids and correct any misbehavior, or they may start hating you as time goes on, especially if you and dad argue a lot.
The ex may resent you for other reasons. Many of us women can be insecure, especially after a break-up or divorce because that's when we're feeling most vulnerable. If that's the case, it may not matter what you say or do, you'll be criminalized by her. Again, Dad needs to make a stand to his kids. He has to mandate they show you respect, and the two of you together need to show that you're a team - that you're together for the long haul.
I can't stress this last point enough. How your partner handles the children's behavior, or misbehavior, could make the difference between them hating you or accepting you, and maybe even becoming part of the group who feel close to their stepmother.