Publishers' rejection letters always say that an author's manuscript “doesn't fit our lists.” It sounds like a euphemistic brush-off but most of the time it's the truth. Different publishers have different personalities, specialties, and market niches, and success for them, and their authors, comes from making a good match. From the editorial point of view, a good match also has to be made between authors, their subjects, and the audience they hope to reach. It's harder than it looks to write the kind of writing that looks natural and conversational!
It's very hard not to take any kind of rejection personally. But if you can throttle back the feelings of frustration, anger, or disappointment, and consider the possibility that the explanation in the letter -- if any -- might actually be an experienced person's honest assessment of the gap between what you offer and what they need, you'll know how to strengthen your case next time.