Not only the code styles, but also the philosophies behind the libraries differ slightly. SFML intends to be beginner-friendly, and its API mostly consists of well-known C++ features (classes, some polymorphism and very few templates). Thor on the other hand sometimes requires knowledge about advanced C++11 features (smart pointers, functionals, ...) -- but once you know them, you can achieve a lot with very few code.
Furthermore, the separation of libraries has the advantage that different approaches can be used to build extensions based on core SFML. For example, some people prefer the simple AnimatedSprite from the Wiki to animate sprites, while others need the genericity and features of thor::Animator. Or you can build a simple resource manager like ResourceHolder from the SFML book, or use a quite sophisticated one conceived for a few special cases, such as thor::ResourceCache.
It would not only be a hard (and considering API stability, potentially irreversible) decision to choose one specific extension rather than the other, but it would also increase the amount of SFML code to be maintained, slowing down overall development. It's crucial that SFML provides a basic abstraction layer that hides low-level libraries and OS-specific APIs, so that other developers can build their high-level extensions on top of it.