Why did he and his wife treat her so poorly? 

I hate to see adults downplay the importance of their relationship with the children in their lives. I’m not talking about a random family member a kid sees once or twice a year: I’m talking about grown-ups who actively take a parental role in their kids’ lives and don’t fully accept the responsibility that entails. They try to use adult terms to justify why they don’t care enough about their stepchildren, claiming that because they’re not biologically related to them, they don’t have an obligation to care for them. Sure, they might not have a legal obligation to pay for the life of a child they didn’t mother or father, but that’s not what’s keeping them up at night. What’s keeping them up at night is knowing the kid worships them and wants their approval more than anything, yet they aren’t doing anything to maintain that relationship. 

Parents should treat their kids equally, but they can’t always do that. There are good reasons not to treat your kids the same all the time. One of your kids might need a lot more support to function on a basic level, so you have to give them more attention than a kid who can function well on their own. The problem is that the «independent» kid can often feel left behind and forgotten by their parents, and that is no good. Sometimes, the kid who is getting all the support doesn’t necessarily «need» that support. He knows he will get it from his parents, and he is willing to act incompetent to get it deliberately. Parents who give their children the opportunity to sink or swim are doing them a much greater favor than those who bail them out at the first sign of friction. 

There’s a difference between giving your kid the chance to acquire some grit and intentionally treating them worse than their siblings. The latter is completely unjustifiable, and the dad in this story is guilty as charged.