Why was Haneda Hideyoshi adopted?

In order to inherit Haneda Kouji's dream.

The son of the capitalist Haneda family died unexpectedly because of the chess genius Haneda Kouji. The Haneda family hoped to adopt Hideyoshi to realize Haneda Kouji's dream of playing chess, so Hideyoshi became the adopted son of the Haneda family in order to inherit Haneda Kouji's dream.

Haneda Hideyoshi is a character in the Japanese anime Detective Conan. He is a chess player by profession. He has won seven Japanese chess championships and is known as a "Taige celebrity".