Answer
Because tert-butyl magnesium bromide is a bulky group it gives only about 1% of the expected addition product along with 99% unreacted cyclohexanone.
Work Step by Step
Because tert-butyl magnesium bromide is a bulkier group it is very difficult to attack at the carbonyl position of the reactant since the bulkier group causes steric hindrance in the compound which makes it an unstable product hence giving only a 1% addition product.
But when D3O+ has been added it gives a stable desired product because it is small in size and can attack carbonyl carbon tert-butyl magnesium bromide. hence give 99% addition product.
