Binet always did have an interest in hypnosis and the workings of the sub-conscious. Around the late 1800's Galton measured intelligence by developing tests around reaction time and sensory motor skills.
Binet was producing work involving ideas on hysteria and hypnosis, after he obtained his doctorate in natural science (he had originally elected to study Law but changed to medicine and science in Germany). His work evolved and changed, as his fascination from subconscious investigation, to the study of high reasoning. As he explored ways of measuring this, and with the birth of his daughters who he examined in their development, he produced a piece of work called 'The Experimental Study of Intelligence'. This was a direct result by utilizing data from what he had learned from his girls.
His work included studies of emotion, memory, attention, and problem solving. This materialised when he started to look at, and was inspired by, the work that the English psychologist Galton (the half cousin of Darwin), as well as the influence of his children and his fascination with the mind.
So, in answer to your question 'what initially motivated binet to develop intelligence test?', you could say that his passion for the ideas on the subconscious, his fascination with his daughter's development and his respect for Galton, lead him to his work in intelligence quotient.
The final push was in 1905, when he created the tests for use by French schools to identify and help children with learning disability. It was designed to produce a simple quotient as a way to summerize their abilities. This was the end result and lead to the his ultimate motivation and conclusion of a life-long work. He died in 1911.