Why did Gottlob Frege not publish the planned third volume of Basic Laws of Arithmetic?

Gottlob Frege never published the projected third volume of his Basic Laws of Arithmetic (original title: Grundgesetze der Arithmetik) primarily due to the discovery of a fundamental contradiction in his logical system, known as Russell’s paradox.12

In 1902, while the second volume was already at the printer, Bertrand Russell informed Frege that his system allowed for the derivation of a contradiction regarding the set of all sets that are not members of themselves. Although Frege attempted to resolve this by modifying one of his axioms in an appendix to the second volume, the discovery effectively marked the end of his productive career. He became deeply embittered by the lack of recognition for his work and the realization that his life's project—founding arithmetic on logic—faced insurmountable difficulties. By the end of his life, he had concluded that his earlier attempt to base arithmetic on logic was mistaken, though he did not pursue this further.123