Why was Bill Finger not credited as a Batman cocreator for many years?

Bill Finger was not credited as a Batman co-creator for more than 75 years primarily due to the terms of the contract between artist Bob Kane and DC Comics.1

Under this agreement, Kane was granted sole credit for the character's creation, despite the fact that Finger was responsible for developing many of the most recognizable elements of the Batman mythos. While ghost artists and writers contributed significantly to the expansion of the Batman universe, Kane’s contract ensured he remained the sole credited creator for decades. It was only after more than 75 years that Finger was officially acknowledged for his foundational role in creating the character alongside Kane.12