It's not a matter of mothers hating their daughters but having a jealousy and rivalry with them. Moms don't feel the same competitiveness with their sons because they don't identify with them as strongly. It's perfectly normal that mothers feel twinges of envy from time to time as it's a basic human emotion. This is especially true when their daughters enjoy experiences in life they didn't: career opportunities, financial successes, travel to exotic places, etc.
Emotionally unhealthy moms, however, feel more than twinges of jealousy. My mother, for instance, felt intense rivalry with my sister and me because she was incredibly insecure. She needed us to make decisions similar to hers in order to validate her life. Not surprisingly, we went to extremes to copy our mother's path so we'd win her love and approval. Sadly, we both became teachers like her even though neither one of us was suited to that profession. My sister got married at the same age as our mom (22), had the same number of children (3), and sent them to the same Catholic schools where our mom sent us. Even though my sister went above and beyond to get my mom's stamp of approval, she never did as my mother alternated between being envious of her and highly critical of her.
When moms get jealous of their daughters, it's best for their daughters to distance themselves. I moved away from my mom (both physically and emotionally) after having my own kids. She had been jealous of the attention I showed them and I felt caught in the middle. When I thought about it, though, I knew it was time for me to grow up, choose my husband and sons, and start a healthy life away from mom. It was the best decision I ever made and contributed greatly to a strong marriage and happy family life.