There is not a hard technical reason, just different goals. WINE is a compatibility layer to run Windows apps, and thus most improvements end up fixing an issue with a particular Windows application. It turns out that most Windows applications are somewhat well-behaved and restrict themselves to calling public win32 APIs and public DLL functions, so implementing 100% coverage of internal APIs wouldn't accomplish much beyond exposing the project to accusations of copyright infringement.

IIRC, there is also US court precedent (maybe Sony v. Connectix?) that protects the practice of reverse-engineering external hardware/software systems that programs use in order to facilitate compatibility. WINE risks losing this protection if they stray outside of APIs known to be used (or are otherwise required) by applications.