He didn't feel that the other tests adequately measured intelligence levels in adults.
The test applied at those time was Binet scale, which differed greatly from The Wechsler–Bellevue Intelligence Scale, released in 1939. Wechsler opposed to the Binet scale items by saying they were not valid for adult test-takers because the items were chosen specifically for use with children. Furthermore, Wechsler said that it was not valid because: “Binet scale's emphasis on speed, with timed tasks scattered throughout the scale, tended to unduly handicap older adults." And he believes that a test should focus on the global capacity of a person to act purposefully, to think rationally, and to deal effectively with his environment. So that Wechsler believed that "mental age norms clearly did not apply to adults.