Jeet kune do

Why did Bruce Lee develop jeet kune do?

Bruce Lee had studied wing chun and felt that wing chun did not fully address certain aspects of combat such as grappling, kicking and floor work.
To resolve this, Bruce Lee sought to develop his own martial arts system.

Many modern martial arts focus on certain aspects of combat and neglect other considerations.
Typically, an art is concerned with grappling or striking/kicking.
Broadly speaking, judo, ju jitsu and aikido are grappling arts... whilst karate, wing chun and tae kwon do are striking arts.